Controller for optically-activated gas sensors

ABSTRACT

An Application Specific Integrated Circuit (ASIC) configured to control one or more gas sensors includes a light emitting diode (LED) driver which receives a pulse width modulator (PWM) signal for driving at least one ultraviolet (UV) LED, wherein an output of the at least one UV LED activates the one or more gas sensors; and an amplifier front end and an analog to digital converter (ADC) configured to calibrate an output of an amplifier to remove offsets associated with outputs associated with the one or more gas sensors, wherein calibration of the amplifier occurs after both generation of the PWM signal and entering a steady state by the one or more gas sensors, further wherein the amplifier receives one or more inputs associated with the one or more gas sensors.

TECHNICAL FIELD

The present invention relates to a controller for optically-activatedgas sensors, e.g., sensors which include a semiconductor nanostructureand at least one of metal or metal-oxide nanoparticles functionalizingthe nanostructure and forming a hybrid sensor that enableslight-assisted sensing of a target analyte.

BACKGROUND

Detection of chemical species in air, such as industrial pollutants,poisonous gases, chemical fumes, and volatile organic compounds (VOCs),is vital for the health and safety of communities around the world (seeWatson J and Ihokura K (1999) Special issue on Gas-Sensing Materials,Mater. Res. Soc. Bull. 24:14). The development of reliable, portable gassensors that can detect harmful gases in real-time with high sensitivityand selectivity is therefore extremely important (Wilson D M et al.(2001) “Chemical Sensors for Portable, Handheld Field Instruments,” IEEESensors Journal 1:256-274; Eranna G et al. (2004) “Oxide Materials forDevelopment of Integrated Gas Sensors—A Comprehensive Review/IntegratedGas Sensors—A Comprehensive Review,” Critical Reviews in Solid State andMaterial Sciences 29:111-188).

Due to their small size, ease of deployment, and low-power operation,solid-state thin film sensors are favored over analytical techniquessuch as optical and mass spectroscopy, and gas chromatography forreal-time environmental monitoring (Wilson D M et al. (2001), supra,IEEE Sensor Journal 1:256-274; Shimizu Y and Egashira M (1999) “Basicaspects and Challenges of Semiconductor Gas Sensors,” Mater. Res. Soc.Bull. 24:18; Sze S M (1994) Semiconductor Sensors 1^(st) ed, Willey; NewYork). Selectivity, which is a sensor's ability to discriminate betweenthe components of a gas mixture and provide detection signal for thecomponent of interest, is an important consideration for the sensor'sreal-life applicability. Conventional metal-oxide based thin filmsensors, despite decades of research and development (Brattain J B W H(1952) “Surface properties of germanium,” Bell. Syst. Tech. Journal32:1; Azad A M et al. (1992) “Solid-State Sensors: A Review,” J.Electrochem. Soc. 139(12):3690-3704), still lack selectivity fordifferent species and typically require high working temperatures(Meixner H and Lampe U (1996) “Metal oxide sensors,” Sens. and ActuatorsB 33:198-202; Nicoletti S et al. (2003) “Use of Different SensingMaterials and Deposition Techniques for Thin-Film Sensors to IncreaseSensitivity and Selectivity,” IEEE Sensors Journal 3:454-459; Demarne Vand Sanjines R (1992) Gas Sensors-Principles, Operation and Developmentsed. G. Sberveglieri, Kluwer Academic, Netherlands). As such, theusability of such conventional sensors is severely limited and poseslong-term reliability problems.

For a chemical sensor, the active surface area is an important factorfor determining its detection limits or sensitivity. It is known thatthe electrical properties of nanowires (NWs) change significantly inresponse to their environments due to their high surface to volume ratio(Cui Y et al. (2001), supra, Science 293:1289-1292; Zhang D et al.(2004) “Detection of NO ₂ down to ppb levels using individual andmultiple In ₂ O ₃ nanowire devices,” Nano. Lett. 4:1919-1924; Kong J etal. (2000) “Nanotube Molecular Wires as Chemical Sensors,” Science287:622-625; Comini E et al. (2002) “Stable and highly sensitive gassensors based on semiconducting oxide nanobelts,” Appl. Phys. Lett.81:1869). NWs are therefore well suited for direct measurement ofchanges in their electrical properties (e.g. conductance/resistance,impedance) when exposed to various analytes. Substantial research hasdemonstrated the enhanced sensitivity, reactivity, and catalyticefficiency of the nanoscale structures (Cui Y et al. (2001), supra,Science 293:1289; Li C et al. (2003) “In ₂ O ₃ nanowires as chemicalsensors,” Appl. Phys. Lett. 8:1613; Wan Q et al. (2004) “Fabrication andethanol sensing characteristics of ZnO nanowire gas sensors,” Appl.Phys. Lett. 84:3654; Wang C et al. (2005) “Detection of H ₂ S down toppb levels at room temperature using sensors based on ZnO nanorods,”Sens. and Actuators B 113:320-323; Wang H T et al. (2005)“Hydrogen-selective sensing at room temperature with ZnO nanorods,”Appl. Phys. Lett. 86:243503; Raible I et al. (2005) “V ₂ O ₅ nanofibers:novel gas sensors with extremely high sensitivity and selectivity toamines,” Sens. and Actuators B 106:730-735; McAlpine M C et al. (2007)“Highly ordered nanowire arrays on plastic substrates for ultrasensitiveflexible chemical sensors,” Nat Mater 6:379-384).

There have been attempts to demonstrate sensors based onnanotube/nanowire decorated with nanoparticles of metal andmetal-oxides. For example, Leghrib et al. reported gas sensors based onmultiwall carbon nanotubes (CNTs) decorated with tin-oxide (SnO₂)nanoclusters for detection of NO and CO (see Leghrib R et al. (2010)“Gas sensors based on multiwall carbon nanotubes decorated with tinoxide nanoclusters,” Sens. and Actuators B: Chemical 145:411-416). Usingmixed SnO₂/TiO₂ included with CNTs, Duy et al. demonstrated ethanolsensing at a temperature of 250° C. (Duy N V et al. (2008) “Mixed SnO ₂/TiO ₂ Included with Carbon Nanotubes for Gas-Sensing Application,” J.Physica E 41:258-263). Balázsi et al. fabricated hybrid composites ofhexagonal WO₃ powder with metal decorated CNTs for sensing NO₂ (BalázsiC et al. (2008) “Novel hexagonal WO ₃ nanopowder with metal decoratedcarbon nanotubes as NO2 gas sensor,” Sensors and Actuators B: Chemical133:151-155). Kuang et al. demonstrated an increase in the sensitivityof SnO₂ nanowire sensors to H₂S, CO, and CH₄ by surfacefunctionalization with ZnO or NiO nanoparticles (Kuang Q et al. (2008)“Enhancing the photon-and gas-sensing properties of a single SnO2nanowire based nanodevice by nanoparticle surface functionalization,” J.Phys. Chem. C 112:11539-11544). ZnO NWs decorated with Pt nanoparticleswere utilized by Zhang et al., showing that the response of Ptnanoparticles decorated ZnO NWs to ethanol is three times higher thanthat of bare ZnO NWs (Zhang Y et al. (2010) “Decoration of ZnO nanowireswith Pt nanoparticles and their improved gas sensing and photocatalyticperformance,” Nanotechnology 21:285501). Chang et al. showed that byadsorption of Au nanoparticles on ZnO NWs, the sensor sensitivity to COgas could be enhanced significantly (Chang S-J et al. (2008) “Highlysensitive ZnO nanowire CO sensors with the adsorption of Aunanoparticles,” Nanotechnology 19:175502). Dobrokhotov et al.constructed a chemical sensor from mats of GaN NWs decorated with Aunanoparticles and tested their sensitivity to N₂ and CH₄ (Dobrokhotov Vet al. (2006) “Principles and mechanisms of gas sensing by GaN nanowiresfunctionalized with gold nanoparticles,” J. Appl. Phys 99:104302). GaNNWs coated with Pd nanoparticles were employed for the detection of H₂in N₂ at 300K by Lim et al. (Lim W et al. (2008) “Room temperaturehydrogen detection using Pd-coated GaN nanowires,” Appl. Phys. Lett.93:072109).

Although such results demonstrate the potentials of thenanowire-nanocluster based hybrid sensors, fundamental challenges anddeficiencies in such prior attempts remain. Most of the results providefor mats of nanowires. Although such mats may increase sensitivity, thecomplex nature of inter-wire conduction makes interpreting the resultsdifficult. Also, room-temperature operation of such previous sensors hasnot been demonstrated, and the selectivity is shown for only a verylimited number of chemicals. Conventional sensor devices require highoperating temperatures (250° C.) and large response times (more than 5minutes). Indeed, such temperature-assisted sensors typically providefor an integrated heater for the device. Further, the reportedsensitivities of such conventional devices were quite low even with longresponse times. Further, such conventional devices typically do notprovide for air as the carrier gas. However, the ability of a sensor todetect chemicals in air is what ultimately determines its usability inreal-life.

Thus, such demonstrations have resulted in poor selectivity of knownchemical sensors, and therefore have not resulted in commercially viablegas sensors. For real-world applications, selectivity between differentclasses of compounds (such as between aromatic compounds and alcohols)is highly desirable. For example, the threat of terrorism and the needfor homeland security call for advanced technologies to detect concealedexplosives safely and efficiently. Detecting traces of explosives ischallenging, however, because of the low vapor pressures of mostexplosives (Moore, D S (2004) “Instrumentation for trace detection ofhigh explosives,” Review of Scientific Instruments 75(8):2499-2512;Yinon J (2002) “Field detection and monitoring of explosives,” TrACTrends in Analytical Chemistry 21(4):292-301; Senesac L. and Thundat T G(2008) “Nanosensors for trace explosive detection,” Materials Today11(3):28-36. Moreover, the difficulty of explosive detection isaggravated by the noisy environment which masks the signal from theexplosive, the potential for high false alarms, and the need todetermine a threat quickly. As such, trained canine teams remain themost reliable means of detecting explosive vapors to date; however, dogsare expensive to train and tire easily.

An ideal chemical sensor would be able to distinguish between theindividual analytes belonging to a particular class of compounds, e.g.detection of the presence of benzene or toluene in the presence of otheraromatic compounds, detection of a particular explosive compound,detection of a particular alcohol, etc. This is extremely challenging asmost semiconductor-based sensors use metal-oxides (such as SnO₂, In₂O₃,ZnO) as the active elements, which are limited due to the non-selectivenature of the surface adsorption sites. The surface/adsorbateinteractions of conventional sensor structures are limited andnon-specific. Thus, conventional sensor devices lack the sameselectivity as their bulk-counterpart devices.

An ideal chemical sensor would be able to distinguish between theindividual analytes belonging to a particular class of compounds, e.g.,detection of the presence of benzene or toluene in the presence of otheraromatic compounds, detection of a particular explosive compound,detection of a particular alcohol, etc. This is extremely challenging asmost semiconductor-based sensors use metal-oxides (such as SnO.sub.2,In.sub.2O.sub.3, ZnO) as the active elements, which are limited due tothe non-selective nature of the surface adsorption sites. Thesurface/adsorbate interactions of conventional sensor structures arelimited and non-specific. Thus, conventional sensor devices lack thesame selectivity as their bulk-counterpart devices.

U.S. Pat. No. 9,476,862, the disclosure of which is incorporated here byreference and which has one or more common inventors with the presentapplication, describes nanostructure sensor devices that address thesedeficiencies of conventional devices by providing a semiconductornanostructure having an outer surface and at least one of metal ormetal-oxide nanoparticle clusters functionalizing the outer surface ofthe nanostructure and forming a photoconductivenanostructure/nanocluster hybrid sensor enabling light-assisted sensingof a target analyte. Additionally, U.S. patent application Ser. No.15/891,709 entitled “Device Having an Array of Sensors on a SingleChip”, the disclosure of which is incorporated here by reference andwhich has one or more common inventors with the present application,describes using one or more nanoparticle gas sensors on a single chip,which sensors are configured to detect at least one type of gas.

The present application focuses on a specific application/implementationof the general type of sensor described in the '862 patent and the '709application to a specific problem set—specifically how to control andcalibrate such sensors to provide more accurate readings of gasconcentrations, etc.

SUMMARY

The present invention is directed to controllers for gas sensors, e.g.,nanoparticle gas sensors.

According to an embodiment, a method for operating an ApplicationSpecific Integrated Circuit (ASIC) controlling one or more gas sensors,includes the steps of powering on the ASIC which generates a pulse widthmodulated (PWM) signal for driving at least one ultraviolet (UV) lightemitting diode (LED), wherein an output of the at least one UV LEDactivates the one or more gas sensors; upon generating the PWM signal,entering a steady state prior to calibrating an output of an amplifier,wherein the amplifier receives one or more inputs associated with theone or more gas sensors; and performing calibration of the amplifier toremove offsets associated with outputs associated with the one or moregas sensors.

According to an embodiment, an Application Specific Integrated Circuit(ASIC) configured to control one or more gas sensors, includes a lightemitting diode (LED) driver which receives a pulse width modulator (PWM)signal for driving at least one ultraviolet (UV) LED, wherein an outputof the at least one UV LED activates the one or more gas sensors; and anamplifier front end and an analog to digital converter (ADC) configuredto calibrate an output of an amplifier to remove offsets associated withoutputs associated with the one or more gas sensors, wherein calibrationof the amplifier occurs after both generation of the PWM signal andentering a steady state by the one or more gas sensors, further whereinthe amplifier receives one or more inputs associated with the one ormore gas sensors.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing/photographexecuted in color. Copies of this patent or patent applicationpublication with color drawing(s) will be provided by the Office uponrequest and payment of the necessary fee.

FIG. 1 , plates (a) and (b), are schematic representations of a GaN(Nanowire)-TiO₂ (Nanocluster) hybrid sensor according to the presentinvention. FIG. 1 , plate (a) shows the sensor in the dark showingsurface depletion of the GaN nanowire, and FIG. 1 , plate (b) shows thesensor under UV excitation with photodesorption of O₂ due to holecapture.

FIG. 2 , plate (a), illustrates graphically the photoresponse of ahybrid device (diameter 300 nm) to 1000 ppm of benzene and toluene mixedin air and nitrogen. FIG. 2 , plate (b) illustrates the response of ahybrid device (diameter 500 nm) to different concentrations of water inair.

FIG. 3 is a schematic representation of depletion in the TiO₂ NC in thepresence of oxygen and water, and its effect on the photogeneratedcharge carrier separation in GaN NW. Circles in valence band indicateholes and circles in conduction band indicate electrons.

FIG. 4 illustrates graphically the photo-response of the GaN/(TiO₂—Pt)device to 1000 μmol/mol of ethanol in air and nitrogen, and to 1000μmol/mol of water in air. The devices did not respond to water innitrogen. The air-gas mixture was turned on at 0 s and turned off at 100s.

FIG. 5 , plate (a) illustrates graphically UV photo-response of theGaN/(TiO₂—Pt) hybrid device to 1000 μmol/mol (ppm) of methanol, ethanol,and water in air, and hydrogen in nitrogen. The air-gas mixture wasturned on at 0 s and turned off at 100 s. FIG. 5 , plate (b) illustratesthe cyclic response of the GaN/(TiO₂—Pt) hybrid device when exposed to2500 μmol/mol (ppm) of hydrogen in nitrogen. The bias voltage for allthe devices was 5 V.

FIG. 6 , plate (a) is a scanning electron microscope (SEM) image of theNW bridge structure according to the present invention. FIG. 6 , plate(b) shows ZnO nanoparticles on the facets of GaN NW. FIG. 6 , plate (c)illustrates graphically current-voltage (I-V) characteristics of thedevice before and after rapid thermal anneal (RTA). FIG. 6 , plate (d)is an x-ray diffraction (XRD) Ω-2Θscan of a 300-nm-thick ZnO film.

FIG. 7 illustrates graphically device response to 500-μmol/mol (ppm) ofmethanol. The inset graph at the bottom left shows the sensitivity oftwo devices toward 500 μmol/mol (ppm) of each isomer of butanol (withDevice 1 shown as the right bar above each isomer, and Device 2 shown asthe left bar above each isomer). The inset graph at the bottom rightshows the response to ethanol, acetone, benzene, and hexane. Sensitivity(S) is given by (I_(g)−I_(α))×100/l_(α), where I_(g) is the devicecurrent in the presence of an analyte in breathing air and/a is thecurrent in pure breathing air, both measured 300 s after the flow isturned on. Percentage standard deviation of the device sensitivity is3.2% based on the five data points collected over a period of 3 days inresponse to the breathing air.

FIG. 8 illustrates graphically device response to different flow ratesof breathing air (plate (a)) and nitrogen gas (plate (b)). The flowrates of the gas are denoted as a=20 sccm, b=40 sccm, c=60 sccm, d=80sccm, and e=100 sccm.

FIG. 9 , plate (a) is a schematic illustration of a nanostructuredsemiconductor-nanocluster hybrid gas sensor according to an embodimentof the present invention. The sensor works with low-intensity light froman LED. The emission wavelength is determined by the semiconductor andmetal-oxide bandgaps. FIG. 9 , plate (b) illustrates schematically anexemplary thin-film device including a semiconductor backbonefunctionalized with TiO₂ on a sapphire substrate. The smoothness of thesubstrate and film after thermal processing is shown in FIG. 9 , plates(c) and (d).

FIG. 10 is a schematic illustration of the mechanism of sensing usingthe disclosed nanocluster-functionalized semiconductor devices. Thesensing is due to the effective separation of photogenerated chargecarriers in the semiconductor backbone caused by surface potentialmodification of the backbone by the nanocluster upon adsorption ofchemicals. The light produces electron-hole pairs in the semiconductor,and also surface defects on the cluster due to photo desorption ofoxygen and water.

FIG. 11 illustrates schematically the epitaxial layer structure utilizedin sensor device fabrication according to an embodiment of theinvention.

FIG. 12 illustrates schematically sensor designs according to thepresent invention, including a sensor having serial architecture (plate(a)), and a sensor having parallel architecture (plate (b)).

FIG. 13 are schematic illustrations of a series architecture design of asensor with four segments, including a top view (plate (a)) and across-section view taken along the dashed line (plate (b)). The sensoroutput is the voltage between the +V_(sensor) and ground pads. TheV_(cal) are the real-time calibration probes for baseline andtemperature drift compensating.

FIG. 14 illustrates graphically a generic sensor calibration curve.Sensitivity S is defined as the slope of the sensor output response vs.analyte concentration plot. The sensor output may be a change incurrent, voltage, or resistance.

FIG. 15 is a schematic illustration of photoexcitation of both themetal-oxide cluster and the GaN backbone using 365 nm light.

FIG. 16 is a schematic illustration showing selectivity tuning using amulticomponent design of nanoclusters. As shown, the target analyte isNO₂ and the interfering chemical is CO₂.

FIG. 17 illustrates graphically depletion depth induced by Ptnanoclusters on GaN and TiO₂ (as calculated by Equation (12) below).

FIG. 18 is a schematic illustration of an integration scheme forstandalone system, showing components at roughly their actual size.

FIG. 19 is a schematic illustration of a hybrid sensor fabricationprocess according to the present invention.

FIG. 20 , plates (a-c), are field-emission scanning electron microscopy(FESEM) images of three different sputtered thickness of TiO₂ coatings:including 2 nm (plate (a)), 5 nm (plate (b)), and 8 nm (plate (c)) ofTiO₂ sputtered on GaN nanowires.

FIG. 21 illustrates graphically an XRD Ω-2Θscan of 150 nm thick TiO₂film deposited on SiO₂/Si substrate at 300° C. and annealed at 650° C.for 45 s in RTA. All indices correspond to the anatase phase [PDF#84-1285].

FIG. 22 illustrates typical morphologies of a 20 nm thick TiO₂ filmsputtered on n-GaN nanowires and annealed at 700° C. for 30 s. FIG. 22 ,plate (a) is a TEM image showing non-uniformly distributed 2 nm to 10 nmdiameter individual TiO₂ particles, with some of the particles marked bywhite circles. FIG. 22 , plate (b) is a high-resolution transmissionelectron microscopy (HRTEM) image of an edge of the GaN nanowire withthe sputtered TiO₂ film. The FFT pattern from the boxed area is shown inexploded view in the upper left inset, indicating 0.35 nm latticefringes which are consistent with a (101) reflecting plane of anatase.

FIG. 23 , plate (a) is a BF-STEM image with 5 nm to 10 nm TiO₂nanoparticles barely visible near an edge of a GaN nanowire, with someof the nanoparticles marked by circles. FIG. 23 , plate (b) is anADF-STEM image of a TiO₂-containing aggregate on the edge of a GaNnanowire. FIG. 23 , plate (c) is an X-ray spectrum of an individual 5 nmTiO₂ particle shown by circled portion ‘A’ in plate (a). FIG. 23 , plate(d) is an EEL spectra recorded on position 1 (tip of the aggregate) andposition 2 (edge of the GaN nanowire), as identified in plate (b),respectively.

FIG. 24 illustrates I-V characteristics of a GaN NW two-terminal devicein the dark at different stages of processing. The inset shows thenanowire device with length 5.35 μm and diameter 380 nm. The scale baris 4 μm. The thickness of sputtered TiO₂ film was 8 nm.

FIG. 25 , plate (a) illustrates graphically the dynamic photocurrent ofa bare GaN NW. FIG. 25 , plate (b) illustrates the dynamic photocurrentof a TiO₂ coated (8 nm deposit) GaN NW. The diameters of both nanowireswere about 200 nm. The applied bias is 5 V in both cases.

FIG. 26 illustrates graphically the dynamic response of a singleGaN—TiO₂ hybrid device to 1000 ppm of toluene. For each cycle, the gasexposure time was 100 s. The inset shows the nanowire device with 8.0 μmlength and diameter 500 nm. The scale bar is 5 μm.

FIG. 27 , plate (a) illustrates the response of a singlenanowire-nanocluster hybrid sensor (inset shows nanowire with diameter500 nm) to 1000 ppm benzene, toluene, ethylbenzene, chlorobenzene, andxylene in presence of UV excitation. FIG. 27 , plate (b) illustrates theresponse of a different sensor (inset shows nanowire with diameter 300nm) to 200 ppb toluene, benzene, ethylbenzene, and xylene with UVexcitation. The total flow in to the chamber was kept constant at 20sccm. The response to air is also shown. The scale bars are 5 μm.

FIG. 28 illustrates graphically a hybrid sensor's photoresponsecharacteristics: FIG. 28 , plate (a) shows the characteristics of thedevice shown in FIG. 27 , plate (a) for 100 to 10000 ppm concentrationrange of toluene; FIG. 28 , plate (b) shows the characteristics of thedevice shown in FIG. 27 , plate (b) for 50 ppb to 1 ppm concentrationrange of toluene.

FIG. 29 illustrates sensitivity plots of a GaN—TiO₂ nanowire-nanoclusterhybrid device (diameter 300 nm) for benzene, toluene, ethylbenzene,chlorobenzene, and xylene. The plot identifies the sensor's ability tomeasure wide range of concentration of the indicated chemicals.

FIG. 30 is an HRTEM image of a GaN NW with TiO₂ sputtered on them, withplate (a) showing the GaN NW before Pt and plate (b) showing after Ptdeposition. Circled areas in plate (a) indicate partially aggregatedpolycrystalline TiO₂ particles on the NW surface and on the supportingcarbon film. Arrows in plate (b) in the inset at the upper left mark Ptclusters decorating a 6 nm diameter particle of titanium. The TiO₂particle exhibits 0.35 nm fringes corresponding to (101) lattice spacingof anatase polymorph. 2 nm to 5 nm thick amorphized surface film areindicated by black arrows.

FIG. 31 illustrate an HAADF-STEM of a GaN NW coated with TiO₂ and Pt.,with plate (a) showing 1 nm to 5 nm bright Pt nanoparticles (shown byarrows) decorating surfaces of a polycrystalline TiO₂ island-like filmand of a GaN nanowire. Medium grey aggregated TiO₂ particles (outlinedby dashed line in plate (a)) are barely visible on a thin carbon supportnear the edge of the nanowire. Plate (b) is a high magnification imageof the supporting film near the edge of the nanowire exhibiting 0.23 nmto 0.25 nm (111) and 0.20 nm to 0.22 nm (200) fcc lattice fringesbelonging to Pt nanocrystallites, with arrows indicating amorphous-likePt clusters of 1 nm and less in diameter.

FIG. 32 illustrates I-V characteristics of the hybrid sensor device atdifferent stages of processing. FIG. 32 , plate (a) shows GaN/(TiO₂—Pt)hybrids; FIG. 32 , plate (b) shows GaN/Pt hybrids. The inset image inplate (b) shows the plan-view SEM image of a typical GaN NWNC hybridsensor. The scale bar in the inset is 4 μm.

FIG. 33 illustrates graphically depletion depth induced by Pt NCs on GaNand TiO₂ as calculated by equation 12.

FIG. 34 illustrates comparative sensing behavior of the three hybridsfor 1000 μmol/mol (ppm) of analyte in air: light gray bar graphs(benzene, toluene, ethylbenzene, xylene, chlorobenzene) representGaN/TiO₂ hybrids, patterned bar graphs (ethanol, methanol, and hydrogen)represent GaN/(TiO₂—Pt) hybrids, and white bar graph (hydrogen)represents GaN/Pt hybrids. Other chemicals which did not produce anyresponse in any one of the hybrids are not included in the plot. Thezero line is the baseline response to 20 sccm of air and N₂. For thisplot the magnitude of the sensitivity is used. The error bars representthe standard deviation of the mean sensitivity values for every chemicalcomputed for 5 devices with diameters in the range of 200 nm-300 nm.

FIG. 35 , plate (a) illustrates graphically the photo-response ofGaN/(TiO₂—Pt) hybrid device to different concentrations of methanol inair. FIG. 35 , plate (b) shows photo-response of the same device todifferent concentrations of hydrogen in nitrogen. The air-gas mixturewas turned on at 0 s and turned off at 100 s.

FIG. 36 , plate (a) is a sensitivity plot of the GaN/(TiO₂—Pt) hybriddevice to ethanol, methanol, and water in air and to hydrogen innitrogen ambient. FIG. 36 , plate (b) shows graphically a comparison ofthe sensitivity of GaN/(TiO₂—Pt) and GaN/Pt devices to differentconcentrations of hydrogen in nitrogen.

FIG. 37 illustrates schematically an exemplary fabrication flow chartfor semiconductor-nanocluster based gas sensors according to the presentinvention.

FIG. 38 , plate (a) is an image of large area etched nanostructures ofGaN on silicon and sapphire substrate formed according to disclosedprocesses such as shown in FIG. 37 . FIG. 38 , plate (b) shows an imageof a nanostructure of GaN on silicon and sapphire using ICP etching andpost-etching surface treatment. This nanostructure forms the backbone ofthe disclosed sensors in disclosed embodiments.

FIG. 39 is an RTEM image of a GaN NW with TiO₂ sputtered on them.Circled portions indicate partially aggregated polycrystalline TiO₂particles on the NW surface and on the supporting carbon film.

FIG. 40 illustrates graphically I-V characteristics of a GaN NWtwo-terminal device at different stages of processing.

FIG. 41 , plate (a) illustrates graphically response of a single,nanowire-nanocluster hybrid sensor to 100 ppb of benzene, toluene,nitrobenzene, nitrotoluene, dinitrobenzene, dinitrotoluene andtrinitrotoluene in the presence of UV excitation. FIG. 41 , plate (b)shows the response of the device to different concentrations oftrinitrotoluene.

FIG. 42 is a sensitivity plot of a GaN—TiO₂ nanowire-nanocluster hybriddevice for benzene, toluene, nitrotoluene, nitrobenzene, DNT, DNB andTNT.

FIG. 43 illustrates sensitivity of two different nanowire-nanoclusterhybrid sensors to 100 ppb of the different aromatic compounds.

FIG. 44 , plate (a), illustrates the dynamic responses of a TiO₂ basedsensor exposed to 250 ppm NO₂ mixed with breathing air under UVillumination and dark at room temperature. Plate (b) illustrates theresponse under UV at mixture of 100 ppm, 250 ppm, and 500 ppm withbreathing air. The inset in plate (b) shows the measured responses underUV as a function of NO₂ concentrations with uncertainty. Sensitivity Sis presented by (I_(g)−I_(α))×100/I_(α), wherein I_(g) is the devicecurrent in the presence of an analyte in breathing air and I_(α) is thecurrent in pure breathing air, both measured 300 s after the flow isturned on.

FIG. 45 illustrates schematically an NO₂ gas sensing mechanism of theTiO₂ sensor under UV illumination: plate (a) shows the mechanism in adark environment with breathing air in; plate (b) shows the mechanismunder UV illumination in breathing air; and plate (c) shows themechanism under UV illumination with mixture of NO₂ and breathing air(all at room temperature).

FIG. 46 illustrates graphically the dynamic response of the TiO₂ basedsensor exposed to 500 ppm NO₂ under UV illumination and under dark atroom temperature.

FIG. 47 , plate (a) illustrates a GIXRD scan of thermally processedultrathin TiO₂ film, and plate (b) illustrates optical properties(bandgap).

FIG. 48 , plate (a) illustrates schematically a SnO₂—Cu nanocluster CO₂sensor. Plates (b) and (c) are AFM images of the SnO₂—Cu nanocluster CO₂sensor.

FIG. 49 illustrates the dynamic responses of the SnO₂—Cu based sensorexposed to CO₂ at room temperature at concentrations of 1000 ppm and5000 ppm. For each cycle, the gas exposure time was 300 s.

FIG. 50 illustrates graphically the response of the SnO₂ based sensor atdifferent relative humidity (RH) concentrations at room temperature.

FIG. 51 , plate (a) illustrates the dynamic response of a TiO₂ basedsensor exposed to methanol at room temperature and at a concentration of500 ppm. Plate (b) illustrates the dynamic response of a ZnO basedsensor exposed to benzene at room temperature and a concentration of 500ppm. Plate (c) illustrates the dynamic response of the ZnO based sensorexposed to hexane at room temperature and a concentration of 100 ppm.

FIG. 52 illustrates graphically the dynamic responses of a ZnO—Pd—Agbased sensor exposed to H₂ at room temperature.

FIG. 53 illustrates schematically an exemplary layout of on chipelements of a sensor device in accordance with the present invention.

FIG. 54 illustrates schematically a micro-heater embedded into a sensordevice in accordance with disclosed embodiments of the presentinvention.

FIG. 55 illustrates the temperature profile of 50 μm microheater madefrom a Ti/Ni metal stack MH recorded at 5 V bias voltage (plate (a)) and10 V bias voltage (plate (b)).

FIG. 56 illustrates graphically the dynamic response of a functionalizedGaN sensor device for sensing H₂S (concentration 50 ppm) in dry air.

FIG. 57 illustrates graphically the dynamic response of a functionalizedGaN sensor device for sensing NO₂ (concentration 500 ppm) in dry air(22° C., relative humidity 0-5%).

FIG. 58 illustrates graphically the dynamic response of a functionalizedGaN sensor device for sensing SO₂ (concentration 10 ppm) in dry air (22°C., relative humidity 0-5%).

FIG. 59 illustrates graphically the dynamic response of a functionalizedGaN sensor device for sensing CO₂ (concentration 5000 ppm) in dry air(22° C., relative humidity 0-5%).

FIG. 60 illustrates an exemplary sensor module in accordance with thepresent invention.

FIG. 61 shows a gas sensor system;

FIG. 62 shows a nanowire photoconductive chemiresistor;

FIG. 63 illustrates a sensor according to an embodiment;

FIG. 64 shows a multi-gas sensor according to an embodiment;

FIG. 65 a flowchart of a process which results in a single package gassensor according to an embodiment;

FIGS. 66A and 66B show a top and bottom portion of a single gas sensorpackage prior to assembly completion according to an embodiment;

FIG. 67 shows a single package gas sensor according to an embodiment;

FIG. 68 shows a sketch of a portion of the single package gas sensoraccording to an embodiment;

FIG. 69 shows another sketch of a portion of the single package gassensor according to an embodiment;

FIG. 70 shows another sketch of a portion of the single package gassensor according to an embodiment;

FIG. 71 shows an example of a desired placement of an LED die and gassensor die on a sensor die and a representation of a gas ingresslocation according to an embodiment;

FIG. 72 shows another example of a desired placement of the LED die andgas sensor die on the sensor die and the representation of the gasingress location according to an embodiment;

FIG. 73 show an example of an undesired placement of an LED die and gassensor die on a sensor die and a representation of a gas ingresslocation;

FIG. 74 shows another example of an undesired placement of the LED dieand gas sensor die on the sensor die and the representation of the gasingress location;

FIG. 75 shows a flowchart of a method for assembling a single packagegas sensor according to an embodiment;

FIG. 76 is a block diagram of an ASIC controller for controlling gassensors according to an embodiment;

FIG. 77 is a block diagram of a portion of the ASIC controller of FIG.76 shown in more detail;

FIG. 78 is another block diagram of a portion of the ASIC controller ofFIG. 76 shown in more detail; and

FIG. 79 is a flow chart illustrating a method according to anembodiment.

DETAILED DESCRIPTION

The present invention is directed to controllers for sensor devices,e.g., sensors including a semiconductor nanostructure, such as a microor nanodevice, or nanowire (NW), having a surface functionalized ordecorated with metal or metal-oxide nanoparticles or nanoclusters. Whenmetal/metal-oxide nanoparticles selected according to the disclosedmethods are placed on the surface of a nanostructure, significantchanges result in the physical properties of the system. Thenanoparticles increase the adsorption of chemical species by introducingadditional adsorption sites, thereby increasing the sensitivity of theresulting system. Further embodiments describe single package gassensors and controllers for such sensors. Before discussing singlepackage gas sensors and controllers (see headers below), a detaileddiscussion of exemplary sensors which can be packaged and/or controlledin accordance with those embodiments will first be discussed(reproduced, at least in large part, from the above cited '862 patent).

The metal or metal-oxide nanoparticles may be selected to act ascatalysts designed to lower the activation energy of a specificreaction, which produces active radicals by dissociating the adsorbedspecies. These radicals can then spill-over to a semiconductor structure(see Sermon P A and Bond G C (1973) “Hydrogen Spillover,” Catal. Rev.8(2):211-239; Conner W C et al. (1986) “Spillover of sorbed species,”Adv. Catal. 34:1), where they are more effective in charge carriertransfer. Further, the selected nanoparticles modulate the currentthrough the nanowire through formation of nanosized depletion regions,which is in turn a function of the adsorption on the nanoparticles.Nanoparticles or nanoclusters suitable for the present invention includevirtually any metal-oxide and/or metal. Thus, it should be understoodthat the present invention is not limited to the particular exemplarymetal-oxides and/or metals disclosed in the various embodiments andexamples herein.

According to one embodiment, nanowire-nanocluster hybrid chemicalsensors were realized by functionalizing n-type (Si doped) galliumnitride (GaN) NWs with TiO₂ nanoclusters. The sensors selectively sensebenzene and related aromatic environmental pollutants, such as toluene,ethylbenzene, and xylene (sometimes referred to as BTEX). GaN is awide-bandgap semiconductor (3.4 eV), with unique properties (Morkoç H(1999) Nitride Semiconductors and Devices, Springer series in MaterialsScience, Vol. 32, Springer, Berlin). Its chemical inertness andcapability of operating in extreme environments (high-temperatures,presence of radiation, extreme pH levels) is thus suitable for thedisclosed sensor design. TiO₂ is a photocatalytic semiconductor with abandgap energy of 3.2 eV (anatase phase). Photocatalytic oxidation ofvarious organic contaminants over titanium dioxide (TiO₂) has beenpreviously studied (see Mills A and Hunte S L (1997) “An overview of ofsemiconductor photocatalysis,” J. Photochem. Photobiol. A 108:1-35; LuoY and Ollis D F (1996) “Heterogeneous photocatalytic oxidation oftrichloroethylene and toluene mixtures in air: Kinetic promotion andinhibition, time-dependent catalyst activity,” J. Catal. 163:1-11). TheTiO₂ nanoclusters were thus selected to act as nanocatalysts to increasethe sensitivity, lower the detection time, and enable the selectivity ofthe structures to be tailored to organic analytes.

The hybrid sensor devices may be developed by fabricating two-terminaldevices using individual GaN NWs followed by the deposition of TiO₂nanoclusters using radio frequency (RF) magnetron sputtering. The sensorfabrication process employed standard micro-fabrication techniques.X-ray diffraction (XRD) and high-resolution analytical transmissionelectron microscopy using energy-dispersive X-ray and electronenergy-loss spectroscopies confirmed the presence of anatase phase inTiO₂ clusters after post-deposition anneal at 700° C.

A change of current was observed for these hybrid sensors when exposedto the vapors of aromatic compounds (e.g., benzene, toluene,ethylbenzene, xylene, and chlorobenzene mixed with air) under UVexcitation, while they had minimal or no response to non-aromaticorganic compounds such as methanol, ethanol, isopropanol, chloroform,acetone, and 1, 3-hexadiene. The sensitivity range for the notedaromatic compounds, except chlorobenzene, were from about 1% down toabout 50 parts per billion (ppb) at room-temperature. By combining theenhanced catalytic properties of the TiO₂ nanoclusters with thesensitive transduction capability of the nanowires, an ultra-sensitiveand selective chemical sensing architecture is achieved.

As discussed in further detail in Example 1 below, GaN—TiO₂(nanowire-nanocluster) hybrid sensors demonstrated a response tospecific volatile organic compounds mixed with air at ambienttemperature and humidity. In the presence of UV light (e.g., having awavelength in the range of about 10 nm to about 400 nm), these hybridsensor devices exhibited change in the photocurrent when exposed tobenzene, toluene, ethylbenzene, xylene, and chlorobenzene mixed in air.However, gases like methanol, ethanol, isopropanol, chloroform, acetone,and 1, 3-hexadiene exhibited little or no change in the electricalcharacteristics of the devices, thus demonstrating the selectiveresponse of these sensors to the aromatic compounds. Benzene, toluene,ethylbenzene, and xylene were detected by the disclosed sensors at aconcentration level as low as 50 ppb in air. In addition, the disclosedsensor devices are highly stable and able to sense aromatic compounds inair reliably for a wide range of concentrations (e.g., 50 ppb to 1%).

In addition, the disclosed sensors demonstrated highly sensitive andselective detection of traces of nitro-aromatic explosive compounds. Asdiscussed in further detail in Example 5 below, GaN/TiO₂nanowire-nanocluster hybrid sensors detected different aromatic andnitroaromatic compounds at room temperature. For example, the GaN/TiO₂hybrids were able to detect trinitrotoluene (TNT) concentrations as lowas 500 μmol/mol (ppt) in air and dinitrobenzene concentrations as low as10 nmol/mol (ppb) in air in approximately 30 seconds. The notedsensitivity range of the devices for TNT was from 8 ppm down to as lowas 500 ppt. The detection limit of dinitrotoluene, nitrobenzene,nitrotoluene, toluene and benzene in air is about 100 ppb with aresponse time of ≅75 seconds. Devices according to the present inventionexhibited sensitive and selective response to TNT when compared tointerfering compounds like toluene. Thus, the disclosed sensors aresuitable for use as highly sensitive, selective, low-power and smartexplosive detectors, which are relatively inexpensive to manufacture inlarger quantities.

Based on structural analysis, an exemplary mechanism that qualitativelyexplains the hybrid sensor's response to different analytes is shown inFIG. 1 . With regard to the photocatalytic processes on the TiO₂surface, the oxygen vacancy defects (Ti³⁺ sites) on the surface of TiO₂are the active sites responsible for adsorption of species like oxygen,water, and organic molecules (see Yates Jr J T (2009) “Photochemistry onTiO2: mechanisms behind the surface chemistry,” Surf. Sci.603:1605-1612). Interestingly, a relatively defect free TiO₂ surface,generated by annealing in high-oxygen flux, is chemically inactive (Li Met al. (1999) “Oxygen-induced restructuring of rutile TiO ₂(110):formation mechanism, atomic models, and influence on surface chemistry,”Faraday Discuss. 114:245). Experimental studies and simulations revealthat molecular oxygen is chemisorbed on the surface vacancies (Ti³⁺sites), acquiring a negative charge as shown in FIG. 1 , plate (a) (AnpoM et al. (1999) “Generation of superoxide ions at oxide surfaces,” Top.Catal. 8:189-198; de Lara-Castells M P and Krause J L (2003)“Theoretical study of the UV-induced desorption of molecular oxygen fromthe reduced TiO ₂ (110) surface,” J. Chem. Phys. 118:5098). This is dueto the presence of the localized electron density at or near exposedTi³⁺ atoms on the TiO₂ surface (Henderson M A et al. (1999) “Interactionof Molecular Oxygen with the Vacuum-Annealed TiO2(110) Surface:Molecular and Dissociative Channels,” J. Phys. Chem. B 103:5328-5337).Water may also be present on the TiO₂ cluster surface via molecular ordissociative adsorption, producing OH⁻ species on the defect sites (LeeF K et al. (2007) “Role of water adsorption in photoinducedsuperhydrophilicity on TiO ₂ thin films,” Appl. Phys. Lett. 90:181928;Bikondoa O et al. (2006) “Direct visualization of defect-mediateddissociation of water on TiO ₂ (110),” Nat. Mater. 5:189-192).

Although most of the theoretical and experimental studies on oxygen andwater adsorption are done for the (110) surface of rutile phase, thereare studies that suggest that similar adsorption behavior is alsoexpected for the anatase surface (Wahab H S et al. (2008) “Computationalinvestigation of water and oxygen adsorption on the anatase TiO ₂ (100)surface,” J. Mol. Chem. Struct. 868:101-108). The GaN NW has a surfacedepletion region as shown in FIG. 1 , plate (a), which determines itsdark conductivity (Sanford N A et al. (2010) “Steady-state and transientphotoconductivity in c-axis GaN nanowires grown bynitrogen-plasma-assisted molecular beam epitaxy,” J. Appl. Phy.107:034318).

In the presence of UV excitation with an energy above the bandgap energyof anatase TiO₂ (3.2 eV) and GaN (3.4 eV), electron-hole pairs aregenerated both in the GaN NW and in the TiO₂ cluster, as shown in FIG. 1, plate (b). Photogenerated holes in the nanowire tend to diffusetowards the surface due to the surface band bending. This effect ofseparation of photogenerated charge carriers results in a longerlifetime of photogenerated electrons, which in turn enhances thephotoresponse of the nanowire devices. On the TiO₂ cluster surface,however, the photogenerated charge carriers lead to a differentphenomenon. In n-type semiconductor oxides such as TiO₂, the surfaceadsorption produces upward band-bending, which drives the photogeneratedholes towards the surface. The chemisorbed oxygen molecule (O₂ ⁻) andhydroxide ions (OH⁻) can readily capture a hole and desorb as shown inFIG. 1 , plate (b) (Perkins C L and Henderson M A (2001)“Photodesorption and Trapping of Molecular Oxygen at the TiO₂(110)—Water Ice Interface,” J. Phys. Chem. B. 105:3856-3863; Thompson TL and Yates J T Jr. (2006) “Control of a surface photochemical processby fractal electron transport across the surface: O(2) photodesorptionfrom TiO(2)(110),” J. Phys. Chem. B 110:7431-7435). The decrease ofphotocurrent through these hybrid sensors when exposed to 20 sscm of airmay be due to the increase in oxygen concentration at the surface ofTiO₂ clusters, leading to an increase in trapping of photogeneratedholes at the surface. This process results in increased lifetime ofphotogenerated electrons. As these nanowires are n-type, excess negativecharge on the surface of the wire (on the TiO₂ clusters) reduces thenanowire current, thus providing a local-gating effect due to netnegative charge accumulation in the TiO₂ clusters. Thus, thephotoinduced oxygen desorption and subsequent capture of holes byorganic adsorbate molecules on the surface of TiO₂ clusters produces thelocal-gating effect, which is responsible for the sensing action of thedisclosed sensor devices. The adsorbed hydroxyl ions may also trap ahole forming OH· species. Other effects such as diffusion of carriersbetween the clusters and the nanowire may also have a role in thesensing properties of the sensors.

Although some embodiments are described in term of excitation in thepresence of UV light, it should be understood that excitation byradiation of other wavelengths may be more suitable for devices havingother types of metal-oxide and/or metal nanoparticles. For example,excitation in the presence of visible light (i.e., having a wavelengthof between about 380 nm and about 740 nm) is suitable for someembodiments.

The process noted above and shown in FIG. 1 also explains sensorresponse when exposed to N₂ flow, as shown in FIG. 2 , plate (a). In thepresence of 20 sccm of N₂ flow, the photocurrent in the sensorsincreases significantly in comparison with 20 sccm of air flow. In an N₂environment, oxygen is desorbed from the surface vacancy sites bycapturing photogenerated holes, but does not get re-adsorbed, resultingin significant reduction of hole capture. As such, the photogeneratedelectron-hole pairs recombine effectively in the cluster. Thus, thephotocurrent through the nanowire/nanocluster hybrid sensor, which isotherwise increased due to the local-gating effect by the TiO₂ clusters,is absent in an N₂ environment.

In the presence of water in air, the photocurrent through these sensorsrecovers towards the level without air flow, as seen in FIG. 2 , plate(b), indicating a reduction of the hole trapping due to adsorption ofwater on the TiO₂ surface. Water may be adsorbed as a molecule on thedefect sites replacing O₂ (see Herman G S et al. (2003) “ExperimentalInvestigation of the Interaction of Water and Methanol with Anatase-TiO₂(101),” J. Phys. Chem. B 107:2788-2795). With increasing waterconcentration, more defects are filled with water. If the adsorbed waterdissociates and produces OH⁻ species, then it is possible that it willact as hole traps and decrease the photocurrent the same way thephotodesorption of oxygen does. A competition between the molecularwater adsorption (reducing hole capture) and dissociative wateradsorption (increasing hole capture) is possible, with the dominantprocess ultimately determining the photocurrent level in the nanowiresin the presence of water.

The presence of aromatic compounds such as benzene, ethylbenzene,chlorobenzene, and xylene in air reduced the photocurrent (e.g. see FIG.2 , plate (a)). Organic molecules are known hole-trapping adsorbates(see Yamakata A et al. (2002) “Electron-and hole-capture reactions onPt/TiO ₂ photocatalyst exposed to methanol vapor studied withtime-resolved infrared absorption spectroscopy,” J. Phys. Chem. B106:9122-9125). Most aromatic compounds show high affinity forelectrophilic aromatic substitution. The exact mechanism ofphotooxidation of adsorbed organic compounds on TiO₂ is complex.However, it is believed that oxidation occurs by either indirectoxidation via the surface-bound hydroxyl radical (i.e., a trapped holeat the TiO₂ surface) or directly via the valence-band hole before it istrapped either within the particle or at the particle surface (seeNosaka Y et al. (1998) “Factors governing the initial process of TiO2photocatalysis studied by means of in situ electron spin resonancemeasurements,” J. Phys. Chem. B 102:10279-10283; Mao Y et al. (1991)“Identification of organic acids and other intermediates in oxidativedegradation of chlorinated ethanes on titania surfaces en route tomineralization: a combined photocatalytic and radiation chemical study,”J. Phys. Chem. 95:10080-10089). In the presence of air (with residualwater) hydroxyl mediated hole transfer to adsorbates such as benzene,xylene is dominant, whereas in the N₂ environment direct transfer ofvalence band holes to aromatic adsorbates could be possible.

Irrespective of the hole transfer mechanism, the presence of additionalhole traps reduces the sensor photocurrent, as observed in the presenceof benzene mixed with N₂ and air as shown in FIG. 2 , plate (a). Themodel disclosed herein qualitatively explains the observed trends forcompounds tested, such as benzene, ethylbenzene, chlorobenzene, andxylene. However, toluene exhibits a different trend, which may be due toother second order effects other than or in addition to the holetrapping mechanism.

The disclosed mechanism is further validated when comparing ionizationenergies of various compounds tested with the responses generated whenthe sensors are exposed to them (see Table I). The effectiveness of theprocess of hole transfer to the adsorbed organic molecules relates tothe compound's ability to donate an electron (i.e. the lower theionization energy of a compound, the easier for it to donate an electronor capture a hole). The observed sensitivity trend for benzene (lowestsensitivity), ethylbenzene, and xylene (highest sensitivity) correlateswith their ionization energies as shown in Table I, with benzene beingthe highest and xylene the lowest among the three.

TABLE I Physical Properties of Various Compounds Tested Organic CompoundSensitivity Ionization Potential (eV) Chloroform No 11.37 Ethanol No10.62 Isopropanol No 10.16 Cyclohexane Yes 9.98 Acetone No 9.69 BenzeneYes (Min) 9.25 Chlorobenzene Yes 9.07 Toluene Yes 8.82 Ethylbenzene Yes8.77 Xylene Yes (Max) 8.52 1,3-Hexadiene No 8.50

As shown in Table I, the sensitivity trend is consistent for aromatics,given 1,3-Hexadiene produced no response in the sensors. Although mostfunctional groups with either a non-bonded lone pair or p-conjugationshow oxidative reactivity towards TiO₂ (Hoffman M R et al. (1995)“Environmental Applications of Semiconductor Photocatalysis,” Chem. Rev.95:69-96), aromatic compounds are more easily photocatalyzed thanaliphatic ones under the same conditions (Carp O et al. (2004)“Photoinduced reactivity of titanium dioxide,” Prog. Solid St. Chem.32:33-177).

Thus, the metal-oxide nanoclusters (TiO₂) on GaN NWs or nanostructuresdemonstrate the disclosed architecture for highly selective gas sensing.The exemplary sensors are capable of selectively sensing benzene andrelated aromatic compounds at nmol/mol (ppb) level in air atroom-temperature under UV excitation.

According to another embodiment, the specific selectivity of thedisclosed nanowire (or nanostructure)/nanocluster hybrid sensors may betailored using a multi-component nanocluster design. For example,catalytic metals (e.g., platinum (Pt), palladium (Pd), and/or any othertransition metals) are deposited onto the surface of oxidephotocatalysts in order to enhance their catalytic activity. Metalclusters on a metal-oxide catalyst alter the behavior of the metal-oxidecatalyst by any one, or a combination of, the following mechanisms: 1)changing the surface adsorption behavior as metals often have verydifferent heat of adsorption values compared to the metal-oxides; 2)enabling catalytic decomposition of certain analytes on the metalsurface, which otherwise would not be possible on the oxide surface; 3)transporting active species to the metal-oxide support by the spill-overeffect from the metal cluster; 4) generating a higher degree ofinterface states, thus increasing reactive surface area reaction area;5) changing the local electron properties of the metal clusters, such asworkfunction, due to adsorption of gases; and 6) effectively separatingphotogenerated carriers in the underlying metal-oxide. The effect oftransition metal loading such as iron (Fe), copper (Cu), Pt, Pd, andrhodium (Rh) onto TiO₂ has been evaluated for photocatalyticdecomposition of various chemicals in both gas-solid and liquid-solidregimes.

In one implementation, the selectivity of the titanium dioxide (TiO₂)nanocluster-coated gallium nitride (GaN) nanostructure sensor device isaltered by addition of platinum (Pt) nanoclusters. In anotherimplementation, the sensor device includes Pt nanocluster-coated GaNnanostructure. The hybrid sensor devices may be developed by fabricatingtwo-terminal devices using individual GaN NWs or nanostructures followedby the deposition of TiO₂ and/or Pt nanoclusters (NCs) using asputtering technique, as described above.

The sensing characteristics of GaN/(TiO₂—Pt) nanowire-nanocluster (NWNC)hybrids and GaN/(Pt) NWNC hybrids is altered as compared to GaN/TiO₂sensors. The GaN/TiO₂ NWNC hybrids show remarkable selectivity tobenzene and related aromatic compounds with no measurable response forother analytes, as discussed above. However, the addition of Pt NCs toGaN/TiO₂ sensors dramatically alters the sensing behavior, making themsensitive only to methanol, ethanol, and hydrogen, but not to otherchemicals tested, as discussed in further detail in Example 2 below.

The GaN/(TiO₂—Pt) hybrid sensors were able to detect ethanol andmethanol concentrations of 100 nmol/mol (ppb) in air in approximately100 seconds, and hydrogen concentrations from 1 μmol/mol (ppm) to 1% innitrogen in less than 60 seconds. However, GaN/Pt hybrid sensors showedlimited sensitivity only towards hydrogen and not towards any alcohols.All the hybrid sensors are operable at room temperature and arephotomodulated (i.e., responding to analytes only in the presence oflight, e.g., ultra violet (UV) light). The selectivity achieved issignificant from the standpoint of numerous applications requiringroom-temperature sensing, such as hydrogen sensing and sensitive alcoholmonitoring. For example, the dynamic response of an exemplary TiO₂ basedsensor exposed to methanol at room temperature and at a concentration of500 ppm is illustrated in FIG. 51 , plate (a). The disclosed sensorstherefore demonstrate tremendous potential for tailoring the selectivityof the hybrid nanosensors for a multitude of environmental andindustrial sensing applications.

A qualitative understanding of the selective sensing mechanism of thedisclosed sensors may be developed by considering how differentmolecules adsorb on the nanocluster surfaces, and determining the rolesof intermediate reactions in the sensitivity of the sensors. While someof the embodiments, examples and explanation describe the invention interms of NWs, it should be understood that other nanostructures ormicrostructures may be utilized. Accordingly, the present invention isnot limited to sensors including NWs.

The photocurrent in GaN/(TiO₂—Pt) hybrid sensors in the presence of air,nitrogen, and water:

The oxygen vacancy defects (Ti³⁺ sites) on the surface of TiO₂ are the“active sites” for the adsorption of species like oxygen, water, andorganic molecules (Yates Jr J T (2009) “Photochemistry on TiO2:mechanisms behind the surface chemistry,” Surf. Sci. 603:1605-1612;Bikondoa O et al. (2006) “Direct visualization of defect-mediateddissociation of water on TiO ₂(110),” Nat. Mater. 5:189-192). It hasbeen observed that oxygen adsorption on photocatalyst powders such asTiO₂ and ZnO quenches the photoluminescence (PL) intensity, whileadsorption of water produces an enhancement of the PL. Electron-trappingadsorbates, such as oxygen, increase the band-bending of TiO₂, whichfacilitates the separation of photogenerated electron hole pairs in theoxide. Subsequently, the PL intensity is decreased as the photogeneratedcharge carries cannot recombine efficiently. Conversely, in the case ofwater, the band bending is reduced, resulting in an increase in the PLintensity. In explaining the observed behavior of the hybrid sensors,the depletion effect induced by the TiO₂ clusters on GaN NW isconsidered. Considering an inverse relationship, i.e., increase indepletion of the TiO₂ cluster leads to a decrease in the depletion widthin the GaN NW and vice versa, some of the observed sensing behavior isexplained.

As shown in FIG. 3 , when oxygen is adsorbed on the TiO₂ NC surface, thedepletion width in the NC increases, leading to a decrease in thedepletion width in the NW. Adsorption of water, nitrogen, and alcoholproduce the reverse effect: they decrease the depletion width of theTiO₂ NC, leading to an increase in the band-bending on the GaN NW.Increased band-bending in the GaN NW results in an effective separationof charge carriers, leading to an increase in photocurrent through theNW. This qualitatively explains the increase in the photocurrent whenthe hybrid sensor is exposed to water mixed with air or with purenitrogen (see FIG. 4 ). However, the increase in the photocurrent whenexposed to 20 sccm of air flow is not fully explained. Under air flow,more oxygen should adsorb on the NCs, causing an increase in thedepletion width of the cluster. This should have resulted in a decreasein the photocurrent based on our assumption; however, an increase in thephotocurrent is exhibited (FIG. 4 ) when 20 sccm of air is passedthrough the chamber.

In the absence of UV light, the absorption or desorption of chemicalsfrom the cluster surfaces cannot modulate the dark current through thenanowire. In the dark, the surface depletion layer of the GaN NW isthicker compared to under UV excitation (see Mansfield L M et al. (2009)“GaN nanowire carrier concentration calculated from light and darkresistance measurements,” Journal of Electronic Materials 38:495-504).The minority carrier (hole) concentration is also significantly lower.Thus the NCs are ineffective in modulating the dark current through theNW.

Mechanism of Sensing of Alcohols and Hydrogen by GaN/(TiO₂—Pt) NWNCSensors

Adsorption of alcohols (RCH₂—OH) on the TiO₂ surface leads to theiroxidation (Kim K S and Barteau M A (1989) “Reaction of Methanol on TiO₂,” Surface Science 223:13-32). Although there are various mechanisms ofoxidation of adsorbed alcohols on TiO₂ surface, focus is on theoxidation of alcohols by photogenerated holes. The process is describedby the following reactions:

RCH₂—OH(g)

RCH₂—OH(ads)  (Equation 1)

RCH₂—OH(ads)+h ⁺(photogenerated hole)

RCH₂—OH⁺(ads)  (Equation 2)

RCH₂—OH⁺(ads)

RCH—OH^(•)(ads)+H⁺(ads)  (Equation 3)

RCH—OH^(•)(ads)

RCHO(ads)+H⁺(ads)+e ⁻  (Equation 4)

where (ads) and (g) represent adsorbed and gas phase species,respectively. For Equation 4 to proceed in the forward directions, theH⁺ species should be removed effectively. It is possible that from TiO₂NCs the H⁺ species can spill-over on to Pt clusters nearby, where theycan be reduced to form H₂:

2H⁺(ads)+2e ⁻

H₂(g)  (Equation 5)

As H⁺ reduction and hydrogen-hydrogen recombination is weak on the bareTiO₂ surface (Fujishima A et al. (2008) “TiO ₂ photocatalysis andrelated surface phenomena,” Surf. Sci. Rep. 63:515-582), the rate ofalcohol oxidation to aldehyde might be affected by the H⁺ reduction andhydrogen-hydrogen recombination on the Pt NCs. Adsorption of alcoholsand their subsequent oxidation due to trapping of photogenerated holesleads to a decrease in the band bending of TiO₂ NCs. As shown in FIG. 3, this leads to an increase in the NW photocurrent, which is observedfor the GaN/(TiO₂—Pt) sensors when exposed to methanol and ethanol (FIG.4 ). It is likely that the production of H.sub.2 on Pt is the key forsensing alcohols by GaN/(TiO₂—Pt) sensors. Additionally, H₂ on Ptsurface can dissociate and diffuse to the Pt/TiO₂ interface. Atomichydrogen is shown to produce an interface dipole layer, which reducesthe effective work-function of Pt (Du X et al. (2002) “A New HighlySelective H2 Sensor Based on TiO2/PtO-Pt Dual-Layer Films,” Chem. Mater.14:3953-3957). Effective reduction of Pt workfunction also reduces thedepletion width in TiO₂, which according to the model in FIG. 4 , alsoleads to an increase in the photocurrent when these sensors are exposedto alcohols. In the presence of hydrogen in nitrogen, the workfunctionchange of Pt NCs due to hydrogen adsorption is the likely cause for thesensing behavior of these sensor hybrids.

Selectivity of GaN/(TiO₂—Pt), GaN/Pt, and GaN/TiO₂NWNC Hybrid Sensors

A significant finding of the present invention is the change in theselectivity of GaN/TiO₂ hybrid sensors due to the addition of Pt NCs.The observed selectivity behavior of the three hybrids can bequalitatively explained if the heat of adsorption of the analytes onTiO₂ and Pt surfaces is considered as shown in Table II and theirionization energies presented in Table III.

TABLE II Heat of Adsorption for Methanol, Benzene, and Hydrogen on Ptand TiO₂ (Anatase*) Hydrogen Methanol Benzene Surface (kJ/mol) (kJ/mol)(kJ/mol) TiO₂ Negligible 92  64 Pt 100 48 117 *The heat of absorptionvalues for TiO2 rutile surfaces are comparable

TABLE III Ionization Energy of the Analytes (CRC Handbook of Chemistryand Physics, 84th ed.; CRC Press: Boca Raton, FL., 2003): OrganicCompound Ionization Energy (eV) Methanol 10.85 Hydrogen 13.5 Benzene9.25

Referring to Table II, benzene has a higher heat of adsorption on Ptthan on TiO₂. Therefore, benzene will preferentially adsorb on Pt in theTiO₂—Pt cluster. Now, in the absence of Pt, when the benzene is adsorbedon TiO₂ it can interact with the photogenerated charge carriersresulting in the sensing behavior of GaN/TiO₂ devices. However, ifbenzene is adsorbed on Pt (such as in the case of TiO₂—Pt and Ptnanoclusters on GaN) then benzene molecules cannot interact withphotogenerated charge carriers in TiO₂, and therefore are ineffective inproducing any current modulation in the nanowire. Thus, benzene isdetected by GaN/TiO₂ sensor devices, but not by GaN/(TiO₂—Pt) and GaN/Ptsensor devices.

Further, methanol is detected by GaN/(TiO₂—Pt) sensors only, and not byGaN/TiO₂ and GaN/Pt sensors. From Table Ill, methanol (unlike benzene)effectively adsorbs on TiO₂, whether Pt is present or absent (as theheat of adsorption of methanol is higher on TiO₂ than Pt). It isbelieved that methanol on TiO₂ in the absence of Pt does not participatein photogenerated carrier trapping as efficiently as benzene and otheraromatic compounds on the TiO₂ nanoclusters. Referring to Table Ill, theionization energy of methanol, hydrogen, and benzene is shown. Theeffectiveness of the process of hole transfer to the adsorbed organicmolecules is related to the compound's ability to donate an electron(i.e. the lower the ionization energy of a compound, the easier for itto donate an electron or capture a hole). However, in the presence of Ptnanoclusters nearby, methanol adsorption on TiO₂ ultimately leads toformation of H⁺ through photo-oxidation of methanol, and eventually H₂,which is the key molecule for sensing of methanol by (TiO₂—Pt) NCs onGaN NW. A similar mechanism applies for ethanol sensing by theGaN/(TiO₂—Pt) hybrids.

Hydrogen is detected by GaN/(TiO₂—Pt) and GaN/Pt hybrids, and not byGaN/TiO₂ NWNC sensors, and GaN/(TiO₂—Pt) sensors have a higher responseto hydrogen than to alcohols. From Table II, hydrogen has negligibleheat of adsorption on TiO₂, thus GaN/TiO₂ devices are not sensitive tohydrogen. However, in the presence of Pt NCs on TiO₂, hydrogen canadsorb on the Pt NCs. Once adsorbed, hydrogen can modify theworkfunction of Pt, resulting in a change in the photocurrent throughthe nanowire. However, this is not the only mechanism, as that wouldimply that GaN/Pt hybrids should be equally sensitive to H₂. It isbelieved that when hydrogen is adsorbed on the TiO₂—Pt NC, it alsoreduces the TiO₂ surface. Thus, in the presence of only Pt on GaN,workfunction modification of Pt solely produces change in thephotocurrent in the NW. However, in the presence of Pt and TiO₂ NCs,hydrogen adsorption leads to the modulation of the photocurrent in GaNNW, through modulation of Pt workfunction together with the change inthe depletion layer of the TiO₂ NCs, resulting in a larger change of thephotocurrent, thus higher sensitivity.

The faster and larger response of GaN/(TiO₂—Pt) towards H₂ compared tothe alcohols (as shown in FIG. 5 ) is due to the fact that in the caseof alcohols, hydrogen is produced after photo-oxidation of the adsorbedalcohols, which is a two-step process with lower yield. In the case ofH₂ in nitrogen, there is a direct availability of H₂ molecules.

GaN/(TiO₂—Pt) sensors are not sensitive to high carbon-containing (C>2)alcohols such as propanol and butanol. In this regard, it has been shownthat the hydrogen production from the photocatalytic oxidation ofalcohols on TiO₂/Pt surface is related to the polarity of the alcohols(i.e., the higher the polarity of the alcohol the greater the yield ofphotocatalytic hydrogen production) (see Yang Y Z et al. (2006)“Photo-Catalytic Production of Hydrogen Form Ethanol over M/TiO2Catalysts (M=Pd, Pt or Rh),” Applied Catalysis B: Environmental67:217-222). The polarity (Y) is defined as Y=(ε_(s)−1)/(ε_(s)+2), whereε_(s) is the relative permittivity of the solvent. Table IV lists thepolarity of various alcohols tested.

TABLE IV Solvent Polarity of Various Alcohols Solvent Polarity Methanol0.91 Ethanol 0.89 n-Propanol 0.86 i-Propanol 0.85 Butanol 0.84

The relative difficulty of producing hydrogen from highercarbon-containing alcohols (C>2) is believed to be the cause of theGaN/(TiO₂—Pt) sensor's inability to detect alcohols with C greater than2. The sensor's greater response to methanol than ethanol (at least forconcentrations above 500 μmol/mol) is also consistent with thepolarities of the alcohols.

The GaN/(TiO₂—Pt) hybrid sensors are operable at room-temperaturesensing of hydrogen, and thus are suitable for various applications(e.g., industrial production facilities, oil refineries, hydrogenmonitoring in hydrogen-powered vehicles, alcohol monitoring systems forindustrial and law-enforcement purposes, etc.). The disclosed mechanismsand methods may be implemented for achieving other multicomponent NWNCbased sensors. For example, the dynamic response of a ZnO—Pd—Ag basedsensor exposed to H₂ at room temperature is illustrated in FIG. 52 .Through combinations of metal and metal-oxides available, a library ofsensors may be produced, each with precisely tuned selectivity, on asingle chip for detecting a wide variety of analytes in many differentenvironments.

Thus, an inactive semiconductor nanostructure (e.g., NW) surface may befunctionalized with selected analyte-specific active metal-oxidenanoparticles. For example, another embodiment of the present inventionprovides for alcohol sensors using gallium nitride (GaN) nanowires (NWs)functionalized with zinc oxide (ZnO) nanoparticles. The dynamic responseof exemplary ZnO based sensors exposed to benzene (concentration 500ppm) and hexane (concentration 100 ppm) at room temperature is shown inFIG. 51 , plates (b) and (c). The disclosed sensors operate at roomtemperature, are fully recoverable, and demonstrate a response andrecovery time on the order of 100 seconds or less. The sensing isassisted by ultraviolet (UV) light within the 215-400 nm band and withthe intensity of 375 nW/cm² measured at 365 nm.

As discussed above, the conductivity model of GaN nanostructure iscomprised of a conducting channel surrounded by a surface depletionregion, where modulation in the width of the depletion region induces achange in the conductivity of the NW. Similarly, ZnO nanoparticles havea surface depletion layer, which enhances upon exposure to air due toelectron capture by surface-adsorbed oxygen. When UV light is turned on,the photogenerated holes in ZnO assist in removing the adsorbed oxygen,thus releasing the electrons captured by surface oxygen back into ZnO.The photoinduced excess of electrons in the ZnO nanoparticles promotesphotogenerated charge separation in the GaN nanostructure, therebyresulting in increased conductivity. Conversely, there is a reduction inthe number of free electrons in the ZnO nanoparticles when exposed toair, leading to a reduced conductivity. As seen in FIG. 6 , this effectincreases with increasing flow rate of air due to enhanced coverage ofthe device surface with adsorbed oxygen.

The device response to alcohols may be explained by the followinggeneric reaction occurring on the surface of ZnO:

2CH₃OH+O⁻ ₂(absorbed)→2HCHO+2H₂ +e ⁻  (Equation 6)

As shown in FIG. 7 , the exposure to alcohol vapors leads to increaseddevice conductivity due to the removal of adsorbed oxygen. In the caseof exposure to N₂, although there is no surface reaction, N₂ assists indesorption of the oxygen, thus restoring the conductivity, as shown inFIG. 8 .

The disclosed hybrid GaN nanostructure/ZnO nanoparticle devices aresuitable for UV-assisted alcohol sensing at room temperature. Thesedevices are a suitable candidate for making nanosensor arrays because oftheir tunable selectivity, ability to detect the pbb level of analytes,and fast response and recovery time.

The disclosed hybrid chemiresistive architectures utilizingnanoengineered wide-bandgap semiconductor backbone functionalized withmulticomponent photocatalytic nanoclusters of metal-oxides and/or metalsare particularly suitable for larger scale manufacturing techniques,such as for commercial applications. The sensors operate atroom-temperature via photoenabled sensing. A substantial benefit of thedisclosed sensors is the utilization of all standard microfabricationtechniques, thus resulting in economical, multianalyte single-chipsensor solution. By combining the “designer” adsorption properties ofmulticomponent nanoclusters together with sensitive transductioncapability of nanostructured semiconductor backbones, photoenabled, roomtemperature, ultra-sensitive, and highly selective chemical sensors areachieved.

The sub-micron structures may be formed on an epitaxial thin-film grownon non-conductive/semi-insulating substrate using deep UV lithographyand a combination of plasma etching and wet-etching. Such structures arefunctionalized with multicomponent nanoclusters of metal-oxides andmetals using reactive-sputter deposition, as noted above.

Referring to FIG. 9 , an exemplary structure of asemiconductor-nanocluster hybrid sensor is illustrated. Referring toFIG. 9 , plate (a), the sensor may comprise a two-terminal sub-micronwide semiconductor backbone, functionalized with nanoclusters ofmetal-oxides and/or metals. For example, the sensor may include alightly-doped 0.8-0.25 μm wide semiconductor two-terminal structure on anon-conductive substrate (e.g. sapphire) formed using traditional deepUV photolithography and plasma etching. Functionalization is adiscontinuous layer of multicomponent nanoclusters (e.g., eachnanocluster comprising one or more photocatalytic metal-oxidenanoclusters (diameter 20 nm and smaller) and smaller metalnanoparticles (5 nm and smaller) deposited on top of it). Themulticomponent design may include more than one oxide and metal types inthe nanoclusters, and exhibits tailored adsorption properties by virtueof the multicomponent design. The functionalization layer is depositedusing reactive sputtering technique followed by thermal treatment—allstandard semiconductor microfabrication processes. The sensors work withlow-intensity light, such as from an LED. The emission wavelength isdetermined by the semiconductor and metal-oxide bandgaps. FIG. 9 , plate(b) illustrates schematically an exemplary thin-film device including asemiconductor backbone functionalized with TiO₂ on a sapphire substrate.The smoothness of the substrate and film after thermal processing isshown in FIG. 9 , plates (c) and (d).

Surface defects of metal-oxides are the active sites for adsorption ofvarious chemicals. However, at room-temperature the adsorbed oxygen andwater are very stable. This necessitates heating in traditionalmetal-oxides sensors. Most metal-oxides are well-know photocatalysts,with photoexcitation wavelengths in the range of ultraviolet to visible,corresponding to the material bandgap. A disclosed approach uses dynamicsurface-defects generation in the metal-oxide cluster throughillumination, which allows for efficient photodesorption of adsorbedwater and oxygen. This has at least two benefits: 1) low-power,room-temperature operation, which also increases the lifetime of thesensors, and 2) real-time dynamic range modulation by changing theintensity of light (for ppt level detection the intensity of the LED canbe increased as compared to ppm level detection).

The sensor architecture provides for the combination of a crystallinetop-down fabricated semiconductor backbone with a discontinuousnanocluster surface layer. In metal-oxide gas sensors, the resistancechanges due to diffusion and adsorption of gases along the grainboundaries. As the present architecture uses a discontinuous,nano-island like metal-oxide layer, the bottleneck of gas diffusionthrough grain boundaries, as in traditional metal-oxide sensors, is notpresent. This makes the disclosed sensors respond relatively fast ascompared to conventional sensors, and operable at room-temperature.Unlike traditional metal-oxide sensors, the disclosed design providesthat the current is carried by the high-quality, high mobilitysemiconductor backbone, which makes the sensor fast. Also, the absenceof conduction in the nanocluster layer makes the active layer inherentlystable as compared to traditional metal-oxide thin film sensors (e.g.,grain boundary motion, defect generation and propagation, and reductionof the metal-oxide layer is not possible due to the absence of a“closed-circuit”).

Due to the nanocluster layer of the disclosed sensors, designed with aspecific adsorption profile, they are extremely efficient in adsorbingtarget analytes. This enables the design of highly-selective sensors.Two component, three component, four component, or five or morecomponent cluster designs are possible for unprecedented selectivitytailoring.

Most semiconductors have depletion regions associated with them. Thesurface band bending, which is a consequence of the surface depletion,facilitates the diffusion of the photogenerated holes to the surface.This separation of carriers effectively suppresses their recombination.The degree of separation is determined by the surface potentialmodification by the clusters. Such separation of photocarriers increasestheir lifetimes, leading to higher photocurrent and thus sensitivitytowards such surface potential modifications. The processes that enablesensing of different adsorbed molecules with the disclosedmulticomponent nanocluster functionalization is shown schematically inFIG. 10 .

Assuming typical values of the response/recovery times for 500 ppt ofNO₂, from the kinetic theory of gases the flux F of NO₂ arriving on asurface is given by the formula:

$\begin{matrix}{F = \frac{N_{A}P_{partial}}{\sqrt{2\pi{MRT}}}} & \left( {{Equation}7} \right)\end{matrix}$

where N_(A) is the Avagadros' number, M is the average molar weight ofthe molecule, P is the pressure, Tis the temperature, and R is the gasconstant.

For 500 ppt concentration of NO₂ in air, three molecules of NO₂ areimpinging on a 20 nm diameter metal-oxide cluster per second. Now, theresidence time r of an adsorbate at temperature T on a surface is givenby the relation τ=τ0 exp (ΔH_(ads)/RT), where ΔH_(ads) is the heat ofadsorption, and TO is correlated with surface atom vibration (roughly10⁻¹² s). Thus, at 298 K the residence time for NO₂ molecule on WO₃nanocluster is approximately 15 seconds (considering ΔH_(ads) for NO₂ onWO₃ to be 18 kcal/mol). Considering roughly 10²¹ cm⁻³ of defect densityfor typical metal oxides, results in roughly 300 adsorption sites on a20 nm diameter nanocluster. Assuming sticking coefficient of 1, by 110seconds the surface defects are saturated. Thus, response time may beestimated to be in the order to 100 seconds, and recovery time in theorder to 15-30 seconds. Although the design of the nanocluster isdescribed from pure thermodynamic standpoint, other surface kinetics(such as diffusion, desorption) may also be considered.

For fabricating the sensor backbone, un-doped (1×10¹⁶ cm⁻³) to lightlydoped (1×10¹⁷ cm⁻³) semiconductor epitaxial layer (1 μm thick) onsapphire/insulating/semi-insulating substrates may be utilized, as shownin FIG. 11 . Lower doping is needed for the sensors to be photo enabled.The thickness of buffer layer controls the defects arising from latticeand thermal mismatch. Ideally suited layer structures require arelatively thin buffer layer (e.g., about 250 nm) to suppress theparasitic conduction in the buffer layer. Similar designs may also beprovided with other direct gap semiconductors, such as ZnO, InN, AlGaNand virtually any other direct gap semiconductor material.

The design of submicron semiconductor backbone including physical layoutand geometry is described with reference to FIG. 12 . Both serial andparallel architectures for the semiconducting resistive backbone haveunique advantages and disadvantages as the chemiresistor backbone.Serial architecture has higher resistance which results in lower-poweroperation, whereas parallel architecture produces more robust devicesinsensitive to material quality variation in the individual sections.However, the calculation will show that the response R is the same forboth serial and parallel architecture:

$\begin{matrix}{R = \frac{R_{analyte} - R_{air}}{R_{air}}} & \left( {{Equation}8} \right)\end{matrix}$

wherein R_(analyte) and R_(air) are the resistances in presence ofanalyte and in air, respectively. However, the resolution of the sensor(i.e., smallest change in concentration it can measure as required forproposed large dynamic range sensors) is greater in a serialarchitecture.

The series sensor element provides for a meander shape, with integratedpassive sections as real-time calibration elements. An exemplary designis shown in FIG. 13 , plate (a). The surface area-volume ratio for thisstructure is roughly 3.1. The sidewalls of the backbone may beintentionally angled, such as at 85° as shown in FIG. 13 , plate (b).This ensures uniform coverage of the nanoclusters on the sidewalls ofthe structure, and also ensures uniform photoexcitation of thesemiconductor backbone. The device is biased by a standard three dcvoltage source (two AA batteries in series) and the sensor output is thevoltage measured between the pads+V_(sensor) and ground. The designprovides various benefits including: 1) high sensitivity and resolution;2) low-power consumption; 3) simplified interface circuit; and 4)ability for real-time base-line drift calibration and temperaturecompensation even in presence of analytes.

Using circuit analysis, it can be shown that Sensitivity S (as definedin FIG. 14 ) may be simplified considering R_(L)<<R as:

$\begin{matrix}{S = {\frac{\text{?} \times \text{?}}{NR}\left\lbrack \frac{\text{?}}{\frac{R}{\Delta R} + 1} \right\rbrack}} & \left( {{Equation}9} \right)\end{matrix}$ ?indicates text missing or illegible when filed

wherein R_(L) is the external low-noise precision load resistance (e.g.,see FIG. 13 , plate (a)), N is the number of segments, R is theresistance without analyte of single segment, and ΔR is the resistancechange of the single segment in presence of the analyte, and V_(dc) isthe dc source voltage.

Thus for higher sensitivity, N should be small, and R_(L) and V_(dc)should be large. However, resolution of a sensor is the smallest changein concentration of the analyte it can measure (it is different fromlowest detection limit), and is often limited by the noise. Consideringonly thermal noise current in the total sensor, the output sensorvoltage noise can be expressed as:

$\begin{matrix}{{\text{?}({noise})} = {R_{L}\sqrt{\frac{4\text{?}T\Delta f}{NR}}}} & \left( {{Equation}10} \right)\end{matrix}$ ?indicates text missing or illegible when filed

wherein k_(B) is the Boltzmann Constant, Tis the temperature, and Δf isthe bandwidth. Considering both Equations 9 and 10, the tradeoff betweenhigh sensitivity and resolution is clear. The effect of N (i.e., numberof segments) on the sensor performances such as sensitivity, detectionlimits, and resolution, may be investigated.

Referring again to FIG. 13 , plate (a), the resistance of the activesensor area may be computed using the formula, neglecting the bends:

$\begin{matrix}{R \approx \frac{\rho \times \left( {4 \times \text{?}} \right)}{h \times \left( {\text{?} + \text{?}} \right)/2}} & \left( {{Equation}11} \right)\end{matrix}$ ?indicates text missing or illegible when filed

wherein ρ=1/(nqμ), ρ is the resistivity, n is the carrier concentration,and μ is the mobility (see also dimensions shown in FIG. 13 , plate(b)).

For example, for the GaN backbone with dimensions shown in FIG. 13 , theactive-area photoresistance under 365 nm excitation from LED is ≈60 kΩ,assuming a mobility of 300 cm²V⁻¹ s⁻¹ and electron concentration of1×10¹⁷ cm⁻³. The device is considered to be excited by low-intensity (10μW/cm2) 365 nm LED. The GaN absorption coefficient α=10⁵ cm⁻¹ for the365 nm photon is assumed. If the sensor is biased with 3 V dc and withan external 10 KΩ resistor, the power dissipation is approximately only40 μW. The sensor power dissipation when in offstate (LED turned off andthe sensor has only dark current) is even lower. The total powerrequirement for the sensor must also include the power required for LEDoperation. There are several low-power UV (365 nm) LEDs (FOX GROUP) thatcould be run by LED drivers. Power dissipation for the LED could be lowas 0.5 mW, if we drive the LED for very low intensity. Using a LEDdriver to control the intensity has an added benefit of the real-timedynamic range configuration.

The simplified chemiresistive architecture lends itself easily tointegration with interface devices as compared to more complex devicessuch as metal-oxide-semiconductor field-effect transistors (MOSFETs).The nano-watt operation amplifier (OP-Amp) TS1001 from TouchstoneSemiconductor is identified, which can provide a gain of 100 whenoperated in single-input voltage amplifier configuration. The Op-Ampoperated from a single AA battery dissipated about 1 μW.

In one implementation, a feature of the present design is the inclusionof the voltage probes (V_(cal)) for calibration of base line drift ofthe photoresistance of the total structure. As the area under thecalibration probes is encapsulated with thick SiO₂, the voltage drop(V_(cal)) for a fixed intensity of illumination through the entirestructure will enable compensation for drift in the baselinephotoresistance arising from persistence photoconductivity ortemperature-induced drift.

Another feature of the present design is the “tailored” adsorptionprofile through the multicomponent nanocluster design, as describedabove. The design provides for suppressing the competitive adsorption ofan interfering chemical on a surface with two different adsorptionprofiles, which is achieved using a primary and a secondary component.

In this regard, FIG. 16 illustrates an exemplary multicomponent designfor the target analyte of NO₂ and for the interfering chemical of CO₂.Adsorption profile for another target analyte or set of analytes alongwith a set of interfering chemicals may alternatively be providedutilizing a similar configuration. The primary metal-oxide component ischosen so that the heat of adsorption of NO₂ on its surface is largecompared to CO₂. The secondary component (e.g., the metal) is chosenwith the heat of adsorption for CO₂ larger than the metal-oxide. ThusNO₂ and CO₂ preferentially adsorb on the metal-oxide and the metal,respectively. When NO₂ is adsorbed on the metal-oxide, it interacts withthe photogenerated charge carriers, producing modulation of thesemiconductor backbone photocurrent, as explained above. However, whenCO₂ is adsorbed on the metal, due to the large concentration ofelectrons, there is minor change in the cluster potential. Considerationof other effects, such as catalytic decomposition on the metal,spill-over from the metal to metal-oxide, and change of metal-workfunction due to adsorption of gases, may also be appropriate.

Due to the highly dispersed nature of the metal phase, even if there isa change in the physical properties of the metals, it has only marginalimpact on the cluster properties. Although the general design principlesare described, the specific designs of the appropriate clusters may befine-tuned for optimal performance and selectivity. For example, Table Vbelow demonstrates possible cluster designs for NO₂ and benzene sensing.Considering the heat of adsorption of NO₂ on WO₃ and Pt, bigger WO₃clusters with much smaller and dispersed phase of Pt may be favorable.Although, adsorption energy for NO₂ is comparable on both WO₃ and Pt,due the higher surface area of metal-oxide clusters, most of NO₂ willadsorb on the WO₃, whereas CO₂ will mostly adsorb on the Pt. For BTEXsensing, the TiO₂/Fe is favorable.

TABLE V Heat of adsorption on different candidates for themulticomponent cluster design. Possible Cluster Designs for NO₂ sensing:NO₂ CO₃ Metal-Oxide Metals (kcal/mol) (kcal/mol) MeO 9.0 3.5 TiO₂ 21.029 WO₃ 18.4 negligible Fe(111) 64.5 69 Pt(111) 19 40.5 Possible ClusterDesigns for Benzene sensing: Benzene CO₂ Metal-Oxide Metals (kcal/mol)(kcal/mol) TiO₂ 15.2 29 Fe (111) 22 69

Note that the values in Table V are average adsorption energies at roomtemperature for low adsorbate coverage. The values are collected fromexperimental results (temperature programmed desorption and calorimetricstudies) and theoretical calculations (such as density function theory).The values shown are for common and stable oxide surfaces. Experimentalheat of adsorption values are dependent on various factors, includingthe morphology and crystal orientation of the surface.

Other design considerations for the nanoclusters include:

1) Bandgap of the oxide: as single wavelength excitation is used forboth photodesorption of surface oxygen and hydroxyl species, and forcreating photocarriers in the semiconductor (e.g. GaN), the bandgap ofthe oxide should be lower or equal to GaN bandgap (as shown in FIG. 15). Candidates are shown in Table VI below.

TABLE VI Bandgaps of Common Metal Oxides Metal-Oxides Bandgap (eV) MgO7.1 TiO ₂ 3.2 WO ₃ 2.8 Fe ₂ O ₃ 2.1 V ₂ O ₅ 2.3 NiO 3.6 Al₂O₃ 7.0Candidates are in bold and underlined, E_(g) < 3.4 eV.

2) Nature of surface defect types: surface defects (i.e. the activeadsorption sites) of metal-oxides are of three types: bronstead,lewis-acid/base sites, and redox sties. Organic compounds such asbenzene predominantly adsorb by dehydgrogenetion (i.e., removal of H+)requiring surface lewis sites. On the other hand, NO₂ predominantlyadsorbs as surface nitrate (NO₃ ⁻), requiring base sites. Mostmetal-oxide surfaces at room-temperature are hydroxylated, and thusphotoexcitation will increase the concentration of one type ofpredominant defects.

3) Redox potentials of the oxide: redox potentials of oxides indicatethe ability of photogenerated carriers to oxidize or reduce any adsorbedmolecule. Depending on whether molecules will be oxidized or reduced onthe surface, they interact with charge carriers differently in theclusters.

4) Stability of the adsorbates: Stability of the adsorbed species is animportant consideration, as it determines the recovery time, andultimately usability of the sensors. As can be seen for Fe, where thevery high adsorption energy might produce very stable NO adsorbedspecies on the surface, rendering the nanoclusters inactive afterexposure to high concentrations of NO₂.

5) Nature of the adsorbed species (molecular or dissociative): nature ofthe adsorbed species determines the photochemical reaction pathways andultimately the sensitivity. Additional multicomponent nanoclusterdesigns for NO₂ and BTEX sensing are shown in Table VII.

TABLE VII Possible designs of nanoclusters Metal-Oxides/Metal TargetAnalyte WO₃/Pt NO₂ TiO₂/Fe BTEX

The use of heterogeneous metal-oxide supported metal catalysts inindustrial production, abatement, and remediation for the past fewdecades has been extensive, and generated an exhaustive body ofliterature that may be readily utilized for nanocluster designsaccording to the present invention. Indeed, some of the systems arewell-understood, so that a desired selectivity outcome may be readilypredicted. The well-known strong metal/support interactions (SMSI)effects in heterocatalysts are different, as the metals are not reducedon the oxides in the disclosed devices.

Computing the size and coverage of the clusters is an importantconsideration, given the size and coverage of the NCs ultimatelydetermines the overall sensitivity of the device. Thus, determination ofthe most effective size and coverage of the clusters is desirable. It isknown that the surface area and relative particle size has a significanteffect on the catalytic properties of metals and metal oxides. However,due to the presence of metals on the metal-oxide clusters, there will besignificant depletion of the metal-oxide clusters. Thus, overly smallmetal-oxide clusters would be substantially depleted and hampereffectiveness, whereas overly large clusters would also result in lowersensitivity. Consideration of the nature of the depletion regions formedby such nano-sized metal clusters on a semiconductor is thereforeprudent.

The classical Schottky model depletion theory cannot predict accuratelythe zero-bias depletion width produced by metallic nanoclusters on asemiconductor. According to Zhdanov's model, the depletion depthassociated with such metal nanoclusters on a semiconductor can beestimated by the following relationship:

$\begin{matrix}{w_{d} = \left( \frac{3r_{c}V_{bi}}{2\pi q^{2}N_{d}} \right)^{1/3}} & \left( {{Equation}12} \right)\end{matrix}$

wherein w_(d) is the depletion width, r_(c) is the radius of thenanocluster, V_(bi) is the built-in voltage for the junction, q is theelementary charge, and N_(d) is the dopant concentration in thesemiconductor.

The plot in FIG. 17 demonstrates the depletion width of TiO₂ clustersdue to Pt particles. It is clear that 4 nm of Pt clusters on 20 nmdiameter TiO₂ clusters would produce depletion of about 5 nm in theTiO₂.

Coverage of the metal-oxide nanocluster functionalization is determinedby the limit of formation of continues metal-oxide film. The coverage isdependent on various parameters such as metal-oxide wetting of thesemiconductor, morphology of phases formed after thermal treatment,etc., and may be verified by SEM imaging. The metal coverage should besparse to ensure only partial depletion of the clusters.

With regard to fabrication, techniques such as wet chemical etching maynot be suitable for etching nanoscale, high aspect-ratio nanostructuresdue to undercutting of the mask and sloped sidewalls. Hence, thedevelopment of a dry etching process with relatively less low damage andprecise-depth control capability is preferred for the fabrication ofnanostructures. Such etching of semiconductor nanostructures isdescribed in further detail in Example 4 below.

Referring to FIG. 18 , the components for an exemplar interface circuitis illustrated. The LED intensity may be controlled by themicrocontroller (MAXQ3213, with a LED driver). By relatively simpledesign change of a selected multicomponent cluster, differentapplications are readily provided. In addition, using wide bandgapmaterial as a backbone enables the sensor to work at elevatedtemperatures, and in presence of radiation and other harsh environmentalconditions.

As shown in Table VIII below, the possible designs of themulti-component nanoclusters are virtually unlimited, resulting in theability to provide sensors for numerous applications.

TABLE VIII Exemplary Designs of Multicomponent Nanoclusters NanoclusterComponents: Semiconductor Metal Oxide: Metal: GaN Titanium OxideTitanium InN Tin Oxide Nickel AlGaN Iron Oxide Chromium Magnesium OxideCobalt Vanadium Oxide Ruthenium Nickel Oxide Rhodium ZnO Zirconium OxideGold InAs Aluminum Oxide Silver Copper Oxide Platinum Zinc OxidePalladium Strontium Oxide Vandium

Thus, in accordance with the disclosed methodologies, sensor devicessuitable for a wide range of applications are achieved. Further, theparticular architecture of the sensor devices may be readily tailoredfor the desired application and associated conditions, as well as one ormultiple active sensor elements configured for sensing particle targets.For example, an exemplary sensor device includes eight individuallyaddressable active sensor elements, as shown in FIG. 53 , which can eachdetect a different target analyte (e.g., various gases). The sensordevice may include an on-chip calibration element for automatic driftcompensation. The sensor device may also include an on-chipmicro-heater, as shown in FIG. 54 , for stabilizing temperature and/orhumidity. The temperature profiles of a 50 μm microheater made from aTi/Ni metal stack MH recorded at 5 V bias voltage and 10 V bias voltageare shown in FIG. 55 , plates (a) and (b), respectively.

Thus, the disclosed sensor devices may comprise various active sensorelements and passive elements for formation of on-chip circuits.Multiple active elements may be provided with a combination of differentfunctionalization to detect multiple gases in a single chip. The chipmay include precise passive elements (elements which have the samesemiconductor backbone but passivated from the environment), forcalibration on the same chip, which has the same temperature coefficientfor current as the active sensor element. Thus, any change due to thetemperature or aging can be a calibrated out using the on-chipcalibration element(s). Using such on-chip components (e.g., see FIG. 53), bridge circuits may be provided directly on the chip, allowing forsensor devices with high resolution.

Although the sensor devices may comprise a micro-heater element as notedabove, such element is not required. The disclosed sensor devices do notneed to be heated for sensing, and are capable of sensing a host ofgases at room temperature. Total power consumption is extremely low(e.g., an exemplary 8 active sensor element device provided for a totalpower consumption about 10 microwatts. Further, the disclosed sensordevices are stable and recoverable even in the presence of corrosivegases (e.g, HCN, CL₂, HCl, etc), and capable of withstanding very highgas concentrations. The sensor devices are also capable of operating inoxygen rich or relatively lean conditions.

In accordance with disclosed embodiments, the active sensor(s) elementsare designed by first selecting a nanoclusters and/or a layer of a basephotocatalytic metal oxide (e.g., TiO₂, V₂O₅, Cr₂O₃, Fe₂O₃, CoO, NiO,CuO, ZnO, ZrO₂, WO₃, MoO₃, SnO₂). Nanoclusters of a catalytic metal(e.g., Ti, V, Cr, Fe, Co, Ni, Cu, Al, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf,Ta, W, Re, Ir, Pt, Au) are then applied on top of the basephotocatalytic metal oxide nanoclusters. Alternatively in otherembodiments, nanoclusters of a second photocatalytic metal oxidedifferent than the base metal oxide are applied on top of the base metaloxide, providing for dual metal oxide functionalizations. Thus, thesensor element comprises a base layer or nanoclusters of a firstmetal-oxide, and nanoclusters of a second metal oxide or metal. Theselection of the particular metal oxide and metal provides for thedesired selectively.

For example, the dynamic response of functionalized GaN NW with selectedmetal oxide for selectively sensing hydrogen sulfide (H₂S) in dry air isshown in FIG. 56 . The response of the functionalized GaN NW for sensingNO₂ in dry air is shown in FIG. 57 . The response of the functionalizedGaN NW for sensing SO₂ in dry air is shown in FIG. 58 . The response ofthe functionalized GaN NW for sensing CO₂ in dry air using the metaloxide sensor devices of the present invention is shown in FIG. 59 .Thus, a wide range of target gases, from reducing to oxidizing to inertgases, is achieved.

A summary of operational and performance specifications of sensingdevices in accordance with disclosed embodiments is set forth in TableIX below:

Response (%) = Analyte Range of Detection (R_(gas) − R_(air)/R_(air))Ammonia 1-100 ppm 15 Chlorine 0.5-10 ppm 212 Hydrogen chloride 1-100 ppm74 Hydrogen cyanide 1-100 ppm 10 Hydrogen sulphide 10-1000 ppm 20Hydrogen 0.5-10%  500 Oxygen 10-30% 40 Carbon dioxide  01.-1% 2 Carbonmonoxide 10-300 ppm 15 Nitrogen dioxide 100-500 ppm 2 Nitric oxide5-1000 ppm 2.6 Methane 50-5000 ppm 9

The disclosed devices are suitable for environmental monitoring, hazmat,large-scale industrial monitoring and control, explosive threatdetection, and other markets where rapid detection of gases andchemicals in air is desired. Compared to conventional sensors, thedisclosed sensors of the present invention are extremely small (e.g., 4mm×4 mm, or 2.5 mm×2.5 mm, or smaller) and inexpensive, exhibit lowpower consumption (e.g., less than 100 microwatts, and in someembodiments less than about 10 microwatts), but capable of sensing alarge dynamic range (e.g., 100 parts per billion to >2%), detect avariety of chemicals under various conditions with no cross-sensitivity(thus minimizing false positives), and exhibit a long operating life. Inaddition, the disclosed sensors of the present invention may bemanufactured using the same manufacturing methodologies utilized forproducing conventional integrated circuits. An exemplary sensor moduleis shown in FIG. 60 , which has dimensions of about 8 cm×6 cm×1 cm, aweight of 0.4 pounds, power consumption requirements for continuousoperation of about 0.2 watts, eight active sensor elements or channelsfor simultaneous measurement of eight different target gases, andincluding a built-in air sampling element with microblower.

The disclosed sensor devices may be installed in residential andcommercial buildings for on-demand ventilation control, resulting in adecrease in energy consumption. The sensors can detect the presence ofharmful VOCs (Benzene, Xylene, and formaldehyde), which are oftenemitted by building materials, paints, and furniture, and are alsoassociated with human metabolism. After detecting an increase in thelevels of targeted harmful chemicals, the ventilation system may beadjusted for safety, comfort and health of the occupants. Alternativelyor in addition, the sensors could monitor CO levels and gas leaks inbuildings for safety. Thus, the disclosed sensor technology may bereadily implemented in indoor monitoring systems, thereby generatinglarge cost savings in terms of energy efficiency, health of theoccupants, and low-maintenance costs.

In case of accidental release of chemicals, the disclosed sensors aresuitable for use by first-responders to detect the presence of chemicalsand associated hazards. Thus, the challenges of a disaster may bemanaged more safely and efficiently. The disclosed hybrid sensortechnology may be implemented in ultra-small, handheld units, whichidentify multiple hazardous materials with low power consumption. Suchdevices would be ideal for first responders.

The disclosed sensors are also suitable for industrial monitoringapplications. For example, the sensors may be used for monitoringdifferent gases for process control in industrial facilities such as oilrefineries, manufacturing plants, etc. They may be installed at variouspoints throughout an industrial facility for point detection for leaksof toxic chemicals. The may also be implemented in personal monitoringdevices for recording personal exposure levels for compliance purposeswith state and federal maximum exposure level regulations. The disclosedtechnology therefore promises unlimited control over the sensor design,thus having the ability to produce sensors for various differentindustries and processes.

Implementations of the disclosed technology for law enforcement andsafety applications are also provided. For example, the disclosedsensors may be utilized in breath analyzers for law-enforcement andindividual use. The hybrid sensors may also be integrated into hand-helddevices (e.g., cell phones) as plug-in modules to existing devices. Forexample, the disclosed sensor may be integrated into a hand-held deviceto enable a user to check his or her blood alcohol level.

Implementations of the disclosed sensor technology are also suitable fordefense and security applications. The sensors may be used for safetymonitoring in public places such as subway/rail stations, airports,public buildings, and in transit systems. For example, the sensors maybe utilized to monitor and detect deliberate release of harmfulchemicals and explosives, thus protecting civilians from attacks. Theymay also be integrated into equipment carried or worn by soldiers fordetection of harmful chemicals, explosives, or other terrorist elements.

Having generally described the invention, the same will be furtherunderstood through reference to the following additional examples, whichare provided by way of illustration and are not intended to be limitingof the present invention unless specified.

EXAMPLES Example 1

Nanowire-nanocluster hybrid chemical sensors were realized byfunctionalizing gallium nitride (GaN) nanowires (NWs) with titaniumdioxide (TiO₂) nanoclusters for selectively sensing benzene and otherrelated aromatic compounds.

Materials and Methods

C-axis, n-type, Si-doped GaN grown by catalyst-free molecular beamepitaxy on Si (111) substrates were utilized. For details of NW growth,see Bertness K A et al. (2008) “Mechanism for spontaneous growth of GaNnanowires with molecular beam epitaxy,” J. Crystal Growth310(13):3154-3158). An exemplary process of sensor fabrication is shownin FIG. 19 . Post-growth device fabrication was done bydielectrophoretically aligning the nanowires on 9 mm×9 mm sapphiresubstrates (see Motayed A et al. (2006) “Realization of reliable GaNnanowire transistors utilizing dielectrophoretic alignment technique,”J. Appl. Phy. 100:114310). The device substrates had 12 nm thick Tialignment electrodes of semi-circular geometry with gaps between themranging from 4 μm to 8 μm. After the alignment of the nanowires, thesamples were dried at 75° C. for 10 min on a hot plate for evaporationof the residual solvent. This was followed by a plasma enhanced chemicalvapor deposition (PECVD) of 50 nm of SiO₂, at a deposition temperatureof 300° C. This passivation layer was deposited to ensure higher yieldfor the fabrication process.

After the oxide deposition, photolithography was performed to defineopenings for the top contact. The oxide in the openings was etched usingreactive ion etching (RIE) with CF₄/CHF₃/O₂ (50 sccm/25 sccm/5 sccm) gaschemistry. The top contact metallization was deposited in anelectron-beam evaporator with base pressure of 10⁻⁵ Pa. The depositionsequence was Ti (70 nm)/Al (70 nm)/Ti (40 nm)/Au (40 nm). The oxidelayer over the nanowires between the end contacts was then etched inbuffered HF etching solution for 15 seconds. A negative resist was usedto protect the end metal contacts from the etching solution.

The TiO₂ nanoclusters were deposited on the exposed GaN NWs using RFmagnetron sputtering. The deposition was done at 325° C. with 50 sccm ofAr flow, and 300 W RF power. The deposition rate was about 0.2 Å/s.Thermal annealing of the complete sensor devices (GaN NW with TiO₂nanoclusters) was done at 650° C. to 700° C. for 30 seconds in a rapidthermal processing system with 6 slpm (standard liter per min) flow ofultrahigh purity Ar. A relatively slow ramp rate of 100° C. per min waschosen to reduce the stress in the metal-nanowire contact area duringheating. The anneal step was optimized to facilitate Ohmic contactformation to the GaN NWs and also to induce crystallization of the TiO₂clusters. Additional lithography was performed to form thick metal bondpads with Ti (40 nm) and Au (160 nm).

The crystallinity and phase analysis of the sputtered TiO₂ films wereassessed by X-ray diffraction (XRD). The XRD scans were collected on aBruker-AXS D8 scanning X-ray micro-diffractometer equipped with ageneral area detector diffraction system (GADDS) using Cu-Kα radiation.The two-dimensional 2Θ-x patterns were collected in the 2Θ=23° to 51°range followed by integration into conventional Ω-Θ scans. Themicrostructure and morphology of the sputtered TiO₂ films used forfabrication of sensors were characterized by high-resolution analyticaltransmission and scanning transmission electron microscopy (HRTEM/STEM)and cold field-emission scanning electron microscopy (FESEM). GaNnanowires with sputtered TiO₂ were deposited onto a lacey carbon filmssupported by Cu-mesh grids and analyzed in a 300 kV TEM/STEM microscope.The instrument was equipped with an X-ray energy dispersive spectrometer(XEDS) and an electron energy-loss spectrometer (EELS) as well asbright-field (BF) and annular dark-field (ADF) STEM detectors to performspot and line profile analyses.

The device substrates, i.e., the sensor chips, were wire-bonded on a 24pin ceramic package for the gas sensing measurements. The devicecharacterization and the time dependent sensing measurements were doneusing an Agilent B1500A semiconductor parameter analyzer. Each sensorchip was placed in a custom-designed stainless steel test chamber ofvolume 0.73 cm³ with separate gas inlet and outlet. The test chamber hada quartz window on top for UV excitation provided by a 25 W deuteriumbulb (DH-2000-BAL, Ocean Optics) connected to a 600 μm diameter opticalfiber cable with a collimating lens at the end for uniform illuminationover the sample surface. The operating wavelength range of the bulb was215 to 400 nm. The intensity at 365 nm measured using an optical powermeter was 375 nW cm⁻². For all the sensing experiments regular breathingair (<9 ppm of water) was used as the carrier gas. A wide range ofconcentrations from 1% to as low as 50 parts per billion (ppb) ofvarious organic compounds were achieved with a specific arrangement ofbubbler and mass flow controllers (MFCs). During the sensormeasurements, the net flow (air+VOC mix) into the test chamber was setto a constant value of 20 sccm. After the sensor devices were exposed tothe organic compounds, they were allowed to regain their baselinecurrent with the air-chemical mixture turned-off, without purging orevacuating the test-chamber.

Results

FIG. 20 shows GaN nanowires with three different nominal thicknesses ofTiO₂ coatings sputtered on them: 2 nm (FIG. 20 , plate (a)); 5 nm (FIG.20 , plate (b)); and 8 nm (FIG. 20 , plate (c)). Rather sparse,well-defined clusters can be seen for both the 5 nm and 8 nmarea-averaged sputtered coatings of TiO₂. The average size of theselarge clusters was about 15 nm. For the 8 nm sputtered coating, thecoverage of the TiO₂ clusters is much denser. However, TEM studiesrevealed the presence of clusters with much smaller diameter (less thanabout 4 nm) on the nanowire surface.

Detection of XRD signal from the TiO₂ decorated GaN NWs was difficultdue to the minuscule size and total volume of TiO₂ nanoclusters. Wetherefore prepared a 150 nm thick TiO₂ film by sputtering it onto a SiO₂coated Si substrate at 300° C. followed by anneal at 650° C. for 45 s inargon. The processing conditions produced an identical morphology as inthe TiO₂ decorated NW case. Referring to FIG. 21 , we identified fromthe XRD that TiO₂ is in the single-phase anatase form. As-deposited TiO₂films were found to be amorphous.

The XRD results agree with the TEM analysis on TiO₂ decorated GaN NWs,which revealed that upon annealing at 700° C. for 30 s, the TiO₂ islandsbecame partially crystalline, as shown in FIG. 22 . Three most commonphases of TiO₂ are anatase, rutile, and brookite. Thermodynamiccalculations predict that rutile is the most stable TiO₂ phase in thebulk state at all temperatures and atmospheric pressure (see Norotsky Aet al. (1967) “Enthalpy of Transformation of a High-Pressure Polymorphof Titanium Dioxide to the Rutile Modification,” Science 158:338;Jamieson J C and Olinger B (1969) “Pressure-temperature studies ofanatase, brookite, rutile, and TiO ₂ (II); A discussion,” Am. Min.54:1477-1480). However, comparative experiments with particle sizeshowed that the phase stability might reverse with decreasing particlesize, possibly due to the influence of surface free energy and surfacestress (Zhang H Z and Banfield J. F (2000) “Understanding polymorphicphase transformation behavior during growth of nanocrystallineaggregates: insights from TiO ₂,” J. Phys. Chem. B 104:3481-3487).Anatase is the most stable phase when the particle size is less thanabout 11 nm, whereas rutile is most stable at sizes greater than about35 nm. Although both rutile and anatase TiO₂ are commonly used asphotocatalyst, anatase form shows greater photocatalytic activity formost reactions (Linsbigler A L et al. (1995) “Photocatalysis on TiO ₂Surfaces: Principles, Mechanisms, and Selected Results,” Chem. Rev.95:735-7; Tanaka K et al. (1991) “Effect of crystallinity of TiO2 on itsphotocatalytic action,” Chem. Phys. Lett. 187:73-76). This is oneconsideration for sputtering nominally 8 nm of TiO₂ for the sensorfabrication.

Although we have sputtered 8 nm of TiO₂ for fabricating the hybridsensors, for the TEM studies 20 nm of TiO₂ coating was utilized. Thethick GaN nanowires prevented acquisition of any TEM diffraction fromthinner TiO₂ coatings. The TEM results presented for 20 nm thick TiO₂was representative of the clusters formed for 8 nm deposited TiO₂ inactual sensors. Typical morphologies of a 20 nm thick TiO₂ filmsputtered on n-GaN nanowires and annealed at 700° C. for 30 seconds areillustrated by TEM data in FIG. 22 . The TEM image in FIG. 22 , plate(a) shows 2 nm to 10 nm diameter individual TiO₂ particles non-uniformlydistributed on the surface of a GaN nanowire. Some of the particles areidentified by circles. Crystallinity of some nanoparticles observed isshown in the HRTEM image in FIG. 22 , plate (b) with nanocrystallites onthe edge of a GaN nanowire with the sputtered TiO₂. The FFT pattern fromthe boxed area is seen in exploded view in the upper left inset image,showing 0.35 nm lattice fringes which are consistent with a (101)reflecting plane of anatase but not available in hexagonal wurtzite-typeGaN crystals.

Referring to FIG. 23 , plate (a), a BF-STEM image shows 5 to 10 nm TiO₂nanoparticles barely visible against the GaN nanowire. An ADF-STEM imageof a TiO₂ island on a GaN nanowire is shown in FIG. 23 , plate (b). Thepresence of TiO₂ was confirmed by analysis of selected areas as well asof individual particles using XEDS and EELS and nanoprobe capabilities.Referring to FIG. 23 , plate (c), the X-ray spectrum of an individual 5nm TiO₂ particle (identified by the marked circle “A” in FIG. 23 , plate(a)) exhibits the TiKα peak at 4.51 keV and the weak Oka peak at 0.523keV. The NKα peak at 0.39 keV and gallium lines (the GaL series at 1.0keV to 1.2 keV) and the CKα peak at 0.28 keV are also identified. EELspectrum acquired at Position “1” marked in FIG. 23 , plate (b) (the tipof a TiO₂-containing aggregate) exhibits the TiL edge at 456 eV and theOK edge at 532 eV and also the CK edge at 284 eV. A reference spectrumrecorded at Position 2 marked in FIG. 23 , plate (b) (an edge of the GaNnanowire) reveals traces of titanium and oxygen with the NK edge at 401eV and the GaL edge at 1115 eV, respectively.

FIG. 24 shows the current-voltage (I-V) characteristics of a GaN NWtwo-terminal device at different stages of processing. The I-V curves ofthe as-deposited devices were non-linear and asymmetric. The currentdecreased when the SiO₂ layer over the NW was etched. However, thecurrent increased with the deposition of TiO₂ nanoclusters. Oxygenadsorption on the bare GaN nanowire surface can introduce surface states(Zywietz et al. (1999) “The adsorption of oxygen at GaN surfaces,” Appl.Phys. Lett. 74:1695), resulting in the decrease of the nanowireconductivity. The devices annealed at 700° C. for 30 seconds showedsignificant changes in their I-V characteristics with a majority of thedevices exhibiting linear I-V curves. This is consistent with the factthat low resistance ohmic contacts to the nitrides require annealing at700° C. to 800° C. (see Motayed A et al. (2003) “Electrical, thermal,and microstructural characteristics of Ti/Al/Ti/Au multilayer ohmiccontacts to n-type GaN,” J. Appl. Phys. 93(2):1087-1094).

FIG. 25 shows the photoconductance of a bare GaN NW device and the TiO₂coated GaN NW device. The NW devices with TiO₂ nanoclusters showedalmost two orders of magnitude increase in the current when exposed toUV light as compared to the similar diameter bare NW devices. Increaseof photoconductance due to surface functionalization has been observedin ZnO nanobelts coated with different polymers (Lao C S et al. (2007)“Giant Enhancement in UV Response of ZnO Nanobelts by PolymerSurface-Functionalization,” J. Am. Chem. Soc. 129:12096-12097). Thisenhancement of photoconductance is often attributed to the separation ofphotogenerated charge carriers by a surface depletion field, therebyincreasing the lifetime of the photogenerated carriers. After the lightis turned off, the photo current decays rapidly, but not to the darkcurrent value, which is likely due to the persistent photoconductivityof the NWs (see Sanford N A et al. (2010) “Steady-state and transientphotoconductivity in c-axis GaN nanowires grown bynitrogen-plasma-assisted molecular beam epitaxy,” J. Appl. Phy.107:034318).

The current through the bare GaN NW devices did not change when exposedto different VOCs mixed in air, even for concentrations as high as fewpercents. On the other hand, the TiO₂ coated hybrid devices respondedeven to the pulses of 20 sccm airflow. This is expected, consideringthat the conduction in most metal-oxides is affected by the presence ofoxygen. The response of the TiO₂ nanocluster-GaN nanowire hybrid sensorto 1000 ppm of toluene in air is illustrated in FIG. 26 . Exposure tothe VOC in the dark had no effect on the hybrid device. However, inpresence of UV excitation, when 1000 ppm of toluene (mixed in air) wasintroduced into the gas chamber, the sensor photocurrent decreaseddramatically to approximately ⅔ of its base value. After 100 seconds ofgas exposure, the gas flow is turned off and the sensor is allowed torecover at room temperature without any additional purging. Therepeatability of the sensing action of these hybrid sensors is evidentfrom FIG. 26 .

Interestingly, the hybrid sensors did not respond when exposed tomethanol, ethanol, isopropanol, chloroform, acetone, and 1,3-hexadiene,even for concentrations as high as several percent. Also, thephotocurrent for these sensors increased with respect to air whenexposed to toluene vapors, whereas for every other aromatic compound,the photocurrent decreased relative to air, as shown in FIG. 27 , plate(a). More than twenty sensor devices were tested, with all exhibitingthe same trend. In addition, the use of toluene from different sourcesresulting in the same sensor behavior. FIG. 27 , plate (b) shows theresponse of a different device for 200 ppb concentrations of the samechemicals. It is clear that even for toluene concentration as low as 200ppb, the relative change in photocurrent is the reverse of that observedwith other chemicals. If the photocurrent in the presence of air forthese sensors is used as their baseline calibration, then we candistinctly identify toluene from other aromatic compounds present in airusing our hybrid devices. The response time is defined as the time takenby the sensor current to reach 90% of the response (I_(f)−I₀) whenexposed to the analyte. The I_(f) is the steady sensor current level inthe presence of the analyte, and I₀ is the current level without theanalyte, which in our case is in the presence of air. The recovery timeis the time required for the sensor current to recover to 30% of theresponse (I_(f)−I₀) after the gas flow is turned off (Garzella C et al.(2000) “TiO ₂ thin films by a novel sol-gel processing for gas sensorapplications,” Sens. and Actuators B: Chemical 68:189-196). The responseand recovery times for ppm levels of BTEX concentrations were ≈60seconds and ≈75 seconds, respectively. The response and recovery timesfor ppb levels of concentrations were ≈180 seconds and ≈150 seconds,respectively. In contrast, conventional nanowire/nanotube sensorsreported in the literature as working at room-temperatures had muchlonger response times in minutes (Leghrib R et al. (2010) “Gas sensorsbased on multiwall carbon nanotubes decorated with tin oxidenanoclusters,” Sens. and Actuators B: Chemical 145:411-416; Balazsi C etal. (2008) “Novel hexagonal WO3 nanopowder with metal decorated carbonnanotubes as NO2 gas sensor,” Sensors and Actuators B: Chemical133:151-155; Kuang Q et al. (2008) “Enhancing the photon- andgas-sensing properties of a single SnO2 nanowire based nanodevice bynanoparticle surface functionalization,” J. Phys. Chem. C112:11539-11544; Lim W et al. (2008) “Room temperature hydrogendetection using Pd-coated GaN nanowires,” Appl. Phys. Lett. 93:072109).Fast response and recovery times indicate fast adsorption anddesorption, which is attributed to the enhanced reactivity of thenanoscale TiO₂ clusters.

The responses of two hybrid devices to different concentrations oftoluene in air are shown in FIG. 28 . FIG. 28 , plate (a) shows thechange of photocurrent of a 234 nm diameter device when exposed totoluene concentrations from 10000 ppm down to 100 ppm. FIG. 28 , plate(b) shows the photocurrent of a sensor device with 170 nm diameter wirefor toluene concentrations from 1 ppm to 50 ppb.

Sensitivity is defined as (R_(gas)−R_(air))/R_(air), where R_(gas),R_(air) are the resistances of the sensor in the presence of thechemical-air mixture and in the presence of air, respectively. Thesensitivity plots of a hybrid device for different VOCs tested are shownin FIG. 29 . The sensitivity plot emphasizes the ability of these hybridsensors to reliably detect BTEX (benzene, toluene, ethylbenzene,chlorobenzene, and xylene), which are common indoor and outdoorpollutants with wide detection range (50 ppb to 1%).

Example 2

The sensing behavior of three NWNC based hybrid sensors was compared: 1)GaN NW coated with TiO₂ NCs (hereafter referred to as GaN/TiO₂ NWNChybrids); 2) GaN NW coated with TiO₂ and Pt multicomponent NCs (i.e.,GaN/(TiO₂—Pt) NWNC hybrids); and 3) GaN NW coated with Pt NCs (i.e.,GaN/Pt NWNC hybrids). It was found that sensors with TiO₂—Ptmulticomponent NCs on GaN NW were only sensitive to methanol, ethanol,and hydrogen. Higher carbon-containing alcohols (such as n-propanol,iso-propanol, n-butanol) did not produce any sensor response. Thesesensors had the highest sensitivity towards hydrogen. Prior to the Ptdeposition, the GaN/TiO₂ NWNC hybrids did not exhibit any response toalcohols, however they detected benzene and related aromatic compoundssuch as toluene, ethylbenzene, xylene, and chlorobenzene mixed with air.The GaN/Pt hybrids only showed sensitivity to hydrogen and not tomethanol or ethanol. The sensitivity of GaN/Pt hybrids towards hydrogenwas lower compared to the GaN/(TiO₂—Pt) hybrids.

Materials and Methods

GaN NWs utilized for this study were c-axis, n-type (Si-doped), grown bycatalyst-free molecular beam epitaxy as described by Bertness K A et al.(2008), supra, J. Crystal Growth 310(13):3154-3158. Post-growth devicefabrication was done by dielectrophoretically aligning the nanowires on9 mm×9 mm sapphire substrates. The details of the device fabrication areset forth in Example 1. After fabrication of two-terminal GaN NWdevices, the TiO₂ NCs were deposited on the GaN NW surface using RFmagnetron sputtering. The deposition was done at 325° C. with 50standard cubic centimeters per minute (sccm) of Ar flow, and 300 W RFpower. The nominal deposition rate was about 0.24 Å/s. Thermal annealingof the complete sensor devices (GaN NW with TiO₂ nanoclusters) was doneat 700° C. for 30 seconds in a rapid thermal processing system. ForTiO₂—Pt composite NCs, the Pt was sputtered using DC sputtering afterannealing of the TiO₂ clusters on GaN NW. The Pt sputtering was donewith an Ar flow of 35 sccm, at a pressure of 1.3 Pa and power of 40 Wfor 10 seconds. For the Pt/GaN devices Pt was sputtered on bare GaN NWsafter annealing the ohmic contacts at 700° C. for 30 seconds. Additionallithography was performed to form thick metal bond pads with Ti (40 nm)and Au (200 nm). The device substrates, i.e., the sensor chips, werewire-bonded on a 24 pin ceramic package for the gas sensingmeasurements.

The microstructure and morphology of the sputtered TiO₂ films used forthe fabrication of the sensors were characterized by high-resolutiontransmission and scanning transmission electron microscopy (HRTEM/STEM),selected-area electron diffraction (SAED), and field-emission scanningelectron microscopy (FESEM). For the TEM characterization, the GaN NWswere dispersed on 10 nm thick carbon films supported by Mo-mesh grids,followed by the deposition of TiO₂ NCs and annealing, and subsequent Ptdeposition. The samples were analyzed in a FEI Titan 80-300 TEM/STEMmicroscope operating at 300 kV accelerating voltage and equipped withS-TWIN objective lenses, which provided 0.13 nm (STEM) and 0.19 nm (TEM)resolution by points. The instrument also had a Gatan CCD imageacquisition camera, bright-field (BF), ADF and high-angle annulardark-field (HAADF) STEM detectors to perform spot, line profile, andareal compositional analyses using an EDAX 300 kV high-performance Si/LiX-ray energy dispersive spectrometer (XEDS).

The as-fabricated sensors were placed in a custom designed gas chamberfor gas exposure measurements. The device characterization and the timedependent sensing measurements were done using an Agilent B1500Asemiconductor parameter analyzer. The gas sensing experiments have beenperformed by measuring the electrical conductance of the devices uponexposure to controlled flow of air/chemical mixture in presence of UVexcitation (25 W deuterium bulb operating in the 215 nm to 400 nmrange). For all the sensing experiments with chemicals, breathing air(<9 μmol/mol of water) was used as the carrier gas. For the hydrogensensing we used high-purity nitrogen as the carrier gas. After thesensor devices were exposed to the organic compounds and hydrogen, theywere allowed to regain their baseline current with the air-chemicalmixture turned-off, without purging or evacuating the test-chamber.

Results

Morphological and Structural Characterization of NWNC Hybrids

It was challenging to measure the sizes and shapes of small TiO₂ and Ptparticles on the surfaces GaN NWs from greyscale TEM images due to: a)270 nm to 300 nm thickness of the NWs used in the devices and variationsof thickness and curvature across the structure; b) diffraction contrastinduced particularly by bending of the wires—even similar particlescould appear as having different intensities, while local thicknessvariations of the carbon support film could result in variable contrastaffecting the mean intensity values of the particles; c) overwhelmingdomination of electron diffraction in SAED from the GaN NW over thediffraction from TiO₂ and Pt nanoparticles. To overcome these problems,TEM imaging was conducted under minimal beam intensity conditions closeto the Scherzer defocus at highest available accelerating voltage of 300kV using both stationary beam (bright-field TEM/SAED, phase-contrasthigh-resolution TEM) and scanning beam (STEM/XEDS) modes. Areas foranalyses were selected near the wire's edges and on the amorphous carbonsupport film in the vicinity of the NWs.

FIG. 30 shows HRTEM micrographs of a GaN NW on a thin amorphous carbonsupport films with TiO₂ coating, before and after the Pt deposition. Thedeposited TiO₂ layer formed an island-like film, where 10 nm to 50 nmpartially aggregated particles (see FIG. 30 , plate (a)) were ofteninterconnected into extended two-dimensional networks. This wasconsistent with SAED and compositional analyses of deposited TiO₂ filmsindicating a mixture of polycrystalline anatase and rutile and of thesame mixture plus fcc Pt nanoparticles (FIG. 30 , plate (b)),respectively. Pt crystalline particles with 1 to 5 nm size were randomlydistributed on the surfaces of TiO₂ islands and sometimes were partiallycoalesced forming elongated aggregates. In the latter case, significantthickness of the GaN NWs made it difficult to visualize TiO₂ depositsdue to the limited contrast difference between TiO₂ and GaN and presenceof multiple heavy Pt particles. In spite of these limitations, detailedHRTEM and HR-STEM observations revealed 0.35 nm (101) hcp latticefringes belonging to anatase (see FIG. 30 , plate (b), upper left inset)and 0.23 nm to 0.25 nm (111) and 0.20 nm to 0.22 nm (200) fcc latticefringes belonging to Pt nanocrystallites, respectively, as well asamorphous-like Pt clusters with diameters around 1 nm or less (see FIG.31 , plates (a) and (b)).

In the FIG. 31 , HAADF-STEM image shows 1 nm to 5 nm diameter bright Ptnanoparticles and barely visible TiO₂ islands (medium grey) randomlydistributed near the edge of the nanowire. The presence of both TiO₂ andPt nanocrystallites was confirmed by the analysis of selected areasusing XEDS nanoprobe capabilities.

Current-Voltage (I-V) Characteristics of NWNC Hybrids in Dark

FIG. 32 shows the I-V characteristics of the GaN/(TiO₂—Pt) and GaN/Pthybrid sensor devices at different stages of processing. A plan-view SEMimage of an exemplary sensor device is shown in the inset of FIG. 32 ,plate (b) for representation purposes. The I-V curves of theas-fabricated GaN NW two-terminal devices were non-linear andasymmetric. A small increase in the positive current after thedeposition of TiO₂ nanoclusters (curve 2) can be attributed to decreasedsurface depletion of the GaN NW due to passivation of surface states,and/or the high temperature deposition (325° C.) of the nanoclustersinitiating ohmic contact formation. The devices annealed at 700° C. for30 s after the deposition of TiO₂ NCs showed significant change in theirI-V characteristics with a majority of the devices exhibiting linear I-Vcurves. Interestingly, Pt NC deposition on TiO₂ coated GaN NWs furtherincreased the conductivity of the nanowire. This is due to the fact thatthe Pt clusters depleted the TiO₂ clusters by removing free electrons.Increased depletion in the TiO₂ clusters due to Pt would decrease TiO₂induced depletion in the GaN NW, leading to an increase in the NWcurrent. With the Pt/GaN hybrids, the current decreases followed by thedeposition of Pt (see FIG. 32 , plate (b)) as expected due to thedepletion region formed in the NW under the metal clusters.

The nature of the depletion region formed by the nano-sized metalclusters on a semiconductor may be determined by Zhdanov's model. FIG.33 shows the calculated zero-bias depletion depth produced in GaN andTiO₂ respectively, as a function of the Pt cluster radius according toEquation (1). For calculating the depletion depth we assumed theeffective conduction band density of states in TiO₂ as 3.0×10²¹ cm⁻³ andpoint-defect related donor concentration as 1.0×10¹⁸ cm⁻³ [43,44]. Theelectron concentration in the GaN NWs was measured to be 1×10¹⁷ cm⁻³ ina separate experiment.

FIG. 33 indicates that even a single Pt NC of 2 nm radius cansignificantly deplete a 10 nm (average size) TiO₂ cluster. The effect ofTiO₂ depletion on GaN NW is difficult to determine as it could beinfluenced by numerous factors including interface states and particlegeometry. Given the very high density of TiO₂ clusters on the NW surface(see FIG. 31 , plate (b)), it is clear that the Pt particles mostlyreside on the surfaces of TiO₂ NCs. However, from FIG. 33 we can seethat when Pt NCs are directly on GaN, they deplete the carriers in aneven larger region in the GaN NW. This qualitatively explains therelatively larger change in current observed when Pt NCs were depositedon bare GaN NWs as compared to the change in current when Pt NC weredeposit on the TiO₂-coated NWs.

Comparative Sensing Behavior of GaN/(TiO₂—Pt), GaN/Pt and GaN/TiO₂ NWNCHybrid Sensors

The photocurrent through the bare GaN NW devices did not change whenexposed to different chemicals mixed in air, even for concentrations ashigh as 3%. In contrast, the TiO₂-coated hybrid devices responded evento the pulses of 20 sccm airflow in the presence of UV excitation. Theresponse of the TiO₂ NC-coated GaN nanowire hybrid sensors to differentconcentrations of benzene, toluene, ethylbenzene, chlorobenzene, andxylene in air is discussed above. The GaN/TiO₂ hybrids showed noresponse when exposed to other chemicals such as alcohols, ketones,amides, alkanes, nitro/halo-alkanes, and esters.

Remarkably, after the deposition of Pt nanoclusters on the GaN/TiO₂hybrids, the sensors were no longer sensitive to benzene and otheraromatic compounds, but responded only to hydrogen, methanol, andethanol. In addition, the GaN/(TiO₂—Pt) hybrids showed no response whenexposed to higher carbon-containing (C>2) alcohols such as n-propanol,iso-propanol, and n-butanol. FIG. 5 shows the change of photocurrent ofa GaN/(TiO₂—Pt) sensor in the presence of 20 sccm air flow of air mixedwith 1000 μmol/mol (ppm) of methanol, ethanol, and water, respectively,and 20 sccm of nitrogen flow mixed with 1000 μmol/mol (ppm) hydrogen.The change in the photocurrent of the sensor when 20 sccm of breathingair is flowing through the test chamber serves as a reference forcalculating the sensitivity of the sensors. The sensitivity is definedas (R_(gas)−R_(air))/R_(air), where R_(gas) and R_(air) are theresistances of the sensor in the presence of the analyte-air mixture andin the presence of air only, respectively (R_(air) is replaced withR_(nitrogen) for hydrogen sensing experiments).

The GaN/TiO₂ hybrids without Pt showed no response to hydrogen and thealcohols. Interestingly, when Pt NC-coated GaN NW hybrids (GaN/Pt) withthe same nominal thickness were tested, they showed very limitedsensitivity only to hydrogen and not to any alcohols. The comparativesummary of the sensing behavior of the three different hybrids arepresented in FIG. 34 .

The response of the GaN/(TiO₂—Pt) NWNC sensor to differentconcentrations of methanol in air is shown in FIG. 35 , plate (a). FIG.35 , plate (b) shows the response to different concentrations ofhydrogen in nitrogen for the same GaN/(TiO₂—Pt) NWNC sensor device. Thesensor response is much higher for hydrogen compared to methanol andethanol. The response time is also much shorter for hydrogen as comparedto methanol, and the sensor photocurrent saturates after initial 20 sexposure.

The response time was defined as the time taken by the sensor current toreach 90% of the response (I_(f)−I₀) when exposed to the analyte. TheI_(f) is the steady sensor current level in the presence of the analyte,and I₀ is the current level without the analyte, which in our case is inthe presence of 20 sccm of air flow. The recovery time is the timerequired for the sensor current to recover to 30% of the response(I_(f)−I₀) after the gas flow is turned off (see Garzella C et al.(2000) Sensors and Actuators B: Chemical 68:189-196). The response timefor hydrogen was ≈60 seconds, whereas the response time for ethanol andmethanol was ≈80 seconds. The sensor recovery time for hydrogen was ≈45seconds and the recovery times for ethanol, methanol was ≈60 seconds and≈80 seconds, respectively. For comparison, Wang et al. demonstrated aconventional ZnO NW-based hydrogen sensor with a response time of 10minutes for 4.2% sensitivity (Wang H T et al. (2005) “Hydrogen-selectivesensing at room temperature with ZnO nanorods,” Appl. Phys. Lett.86:243503).

The sensitivity plot of a GaN/(TiO₂—Pt) hybrid device for the variousanalytes tested is shown in FIG. 36 , plate (a). Note that the lowestconcentration detected for methanol and hydrogen (1 ppm or μmol/mol) isnot the sensor's detection limit, but a system limitation. It can beseen that the sensor is more sensitive to methanol than ethanol forconcentrations ≥1000 μmol/mol (ppm), and the relative sensitivityswitches for concentrations of 500 μmol/mol (ppm) and below. Similarbehavior is observed with twenty unique devices, possibly due todifference in surface coverage of the different alcohols over theconcentration range. FIG. 36 , plate (b) is a comparative plot showingthe sensitivity of GaN/(TiO₂—Pt) and GaN/Pt hybrid sensors to hydrogenin nitrogen. The GaN/Pt hybrid devices showed relatively low sensitivitywith detection limit of 50 μmol/mol (ppm), below which the devicesstopped responding. The gas exposure time was also increased to 200seconds for the GaN/Pt devices to obtain increased response compared to100 seconds for the GaN/(TiO₂—Pt) GaN devices. The sensitivity of theGaN/(TiO₂—Pt) sensors was greater for alcohols and hydrogen whencompared with the same concentrations of water in air, which thusenables their use in high-humidity conditions.

Table X and Table XI compare the performance of the sensor devices ofthe present invention with sensors disclosed in the most recentliterature in terms of operation temperature, carrier gas, lowerdetection limit, and response/recovery times. The comparison indicatesthat the sensors devices of the present invention exhibit an excellentresponse to very low concentrations of analytes (100 ppb for ethanol and1 ppm for hydrogen) at room temperature, with air as the carrier gas.The testing conditions closely resembled real-life conditions, whichunderlines the significance of the disclosed sensors. The response andrecovery times were also lower for the disclosed sensors compared to theother conventional sensors, as shown in Tables X and XI.

TABLE X Performance of GaN/(TiO₂—Pt) NWNC hybrid sensors to ethanol incomparison with conventional sensors Response/ Lower Testing RecoveryTime Detection Limit Carrier Gas Temperature Sensor of Present 80 s/75 s100 ppb with air Room Invention 1% sensitivity⁴ temperature (RT)CNT¹/SnO₂ core shell  1 s/10 s 10 ppm air 300° C. nanostructuresMWCNTs²/NaClO₄/ 20 s/20 s 18,000 ppm air RT polypyrrole Metal-CNThybrids  ~2 min/(recovery 500 ppb with N₂ in a vacuum RT time notreported) sensitivity < 1% test chamber V₂O₅ nanobelts 50 s/50 s 5 ppmair 150° C.-400° C. ZnO nanorods 3.95 min/5.3 min  10 ppm Synthetic air125° C.-300° C. ZnO nanowires 10 s/55 s 1 ppm air 220° C. ITO³ nanowires2 s/2 s 10 ppm air 400° C. SnO₂ nanowires 2 s/2 s 10 ppm air 300° C.¹Carbon nanotubes ²Multiwall carbon nanotubes ³Indium tin oxide⁴Sensitivity values for sensors with lowest limit similar to disclosedresults were compared.

TABLE XI Performance of GaN/(TiO₂—Pt) NWNC hybrid sensors to hydrogen incomparison with conventional sensors Response/ Lower Testing recoverytimes detection limit Temperature Sensor of Present 60 s/45 s 1 ppm withRT Invention sensitivity of 4% CNT films 5 min/30 s    10 ppm RTSWCNT/SnO₂ 2 s/2 s 300 ppm 250° C. Pd/CNTs 5 min/5 min 30 ppm with RTsensitivity of 3% Pd/Si NWs   1 hr/50 min  3 ppm RT Pt doped  2 min/10min 100 ppm 100° C. SnO₂ NWs

The present results indicate the unique ability to tailor theselectivity of NWNC chemical sensors. With infinite combinations ofmetal and metal-oxide composite clusters available, there is a hugepotential for sensor designs targeted for a multitude of applications.

Example 3

Alcohol sensors using gallium nitride (GaN) nanowires (NWs)functionalized with zinc oxide (ZnO) nanoparticles are demonstrated.These sensors operate at room temperature, are fully recoverable, anddemonstrate a response and recovery time on the order of 100 seconds.The sensing is assisted by UV light within the 215-400-nm band and withthe intensity of 375 nW/cm² measured at 365 nm. The ability tofunctionalize an inactive NW surface, with analyte-specific activemetal-oxide nanoparticles, makes this sensor suitable for fabricatingmultianalyte sensor arrays.

Methods and Materials

Si-doped c-axis n-type GaN NWs were grown using catalyst-free molecularbeam epitaxy on Si (III) substrate as described in Bertness K A et al.(2008), supra, J. Cryst. Growth 310(13):3154-3158. The NW diameter andlength were in the ranges of 250-350 nm and 21-23 μm, respectively. TheGaN NWs were detached from the substrate by sonication in isopropanoland dielectrophoretically aligned across the pre-patterned electrodes.The electrodes were fabricated using photolithography followed bydeposition of a metal stack of Ti (40 nm)/AI (420 nm)/Ti (40 nm). Thickbottom electrodes ensure the free suspension of the NWs. For theformation of ohmic contacts to the NW ends, the top metal contacts werefabricated using a metal stack of Ti (70 nm)/AI (70 nm)/Ti (40 nm)/Au(40 nm), as described in A. Motayed et al. (2003), supra, J. Appl. Phys.93(2):1087-1094. Rapid thermal anneal (RTA) was performed at 700° C. for30 seconds in argon atmosphere to promote the formation of ohmiccontacts and to reduce the stress in the thick bottom electrodes.Finally, ZnO nanoparticles were sputter deposited on the NW device withan RF power of 300 W in 60 standard cubic centimeters per minute (sccm)of oxygen and 40 sccm of argon gas flow at room temperature. Depositiontime of 160 seconds was found to be optimal for the formation ofuncoalesced oxide nanoparticles.

The microstructure of the devices was characterized using a scanningelectron microscope (SEM) and X-ray diffraction (XRD). Due to the smallsize of the nanoparticles, the XRD signal from ZnO was not detected.Thus, the analysis was performed on a 300-nm-thick ZnO film sputterdeposited on Si (111) substrate with the assumption that the ZnOcrystallinity is similar for nanoparticles and for thin films depositedat the identical conditions. Current-voltage characteristics of thedevices were also measured to determine the nature of the NW-metalcontacts.

For the gas sensing measurements, a device was placed inside thestainless steel chamber with an inlet and an outlet for the analytevapors. The chamber, with a volume of 0.73 cm³, has a quartz window onthe top to facilitate exposure of the device to UV light. The wavelengthof the light bulb was confined to the range of 215-400 nm; the intensityrecorded at 365 nm was 3.75 nW/cm². The sensor baseline was establishedat a constant flow of 40 sccm of breathing air under illumination. Forsensing experiments, 40 sccm of the mixture of the breathing air andanalyte vapor was passed through the chamber. All sensing measurementswere performed in the presence of UV light and 5-V dc voltage biasapplied across the device terminals. Negligible or no chemiresistiveresponse was observed for all the chemicals in the absence of theillumination.

Results and Properties

FIG. 6 , plate (a) shows a SEM image of a device with a single GaN NWsuspended across the metal electrodes. FIG. 6 , plate (b) shows the ZnOnanoparticles on the facets of a GaN NW. The current-voltagecharacteristics of the device measured before and after RTA are shown inFIG. 6 , plate (c). As shown in FIG. 6 , plate (d), XRD reveals that thesputter-deposited ZnO is crystalline and highly (0002) textured.

Referring to FIG. 8 sensor response to air and nitrogen was evaluated.FIG. 8 , plate (a) shows the device response to the different flow ratesof breathing air. As seen therein, device conductance decreases uponexposure to the breathing air, and the decrease is proportional to theflow rate. Opposite behavior (i.e., an increase in conductivity) isobserved when the device is exposed to nitrogen gas as seen in FIG. 8 ,plate (b).

Referring to FIG. 7 , sensor response to alcohols and other analytes wasevaluated. When exposed to alcohol vapors (methanol, ethanol,n-propanol, isopropanol, n-butanol, and isobutanol), the devices showedan increase in conductivity with maximum sensitivity toward methanol.FIG. 7 shows the device response to 500-μmol/mol (ppm) methanol vapor inbreathing air.

For the isomers of an alcohol, the sensitivity decreases with branchingin the carbon chain. Hence, as shown in FIG. 7 (inset, bottom left), thesensitivity toward isobutanol is less than that toward n-butanol. Asshown in FIG. 7 (inset, bottom right), the devices show a negligibleresponse to possible interfering chemicals such as benzene and hexane,whereas the sensitivity toward 100 μmol/mol (ppm) of ethanol is similarto the sensitivity toward 1000 μmol/mol (ppm) of acetone. Ethanol vaporconcentration down to 100 nmol/mol (ppb) was successfully detected, andthe detection of even lower concentrations is possible with alternativemeasurement setup.

Example 4

A hybrid chemiresistive architecture, utilizing nanoengineeredwide-bandgap semiconductor backbone functionalized with multicomponentphotocatalytic nanoclusters of metal-oxides and metals was demonstrated.These sensors operated at room-temperature via photoenabled sensing.

Etching of Semiconductor Nanostructures

For real-time nanosensors, successful etching of semiconductingnanostructures, which is characterized by smooth surfaces with minimalsub-surface damage and appropriate side-wall profiles, is desired. Thisrequires overcoming the strong chemical bond energy in widegapsemiconductors, and also adjusting the process conditions to overcomeinherent defects in epitaxially grown films on non-native substratesusing heteroepitaxy. Otherwise, an un-optimized etching process mayresult in surface morphologies that include pits and/or pillars.

An Inductively Coupled Plasma-Reactive Ion Etching (ICP-RIE) processwith Cl₂/Ar/N₂ chemistry is provided, with an etch rate of about 100nm/min for GaN. The dry etching process may be optimized using X-rayphotoelectron spectroscopy (XPS), scanning electron microscopy (SEM),photoconductivity measurements, and photoluminescence (PL) measurements.

Fabrication Detail

Prior to dry etching, semiconductor wafer surfaces are treated withstandard RCA cleaning procedures. As a mask for selective etching, a500-nm-thick SiO₂ film is deposited by standard plasma-enhanced chemicalvapor deposition (PECVD). Etching patterns are defined by deep UVlithography using a proximity aligner capable of generating 300 nmfeature sizes. Electron beam deposition of Ni (˜20 nm) followed bylift-off is carried out to complete the formation of mask for the SiO₂etch.

Direct metal-masking of the semiconductor is not done in order to avoidun-intentional doping of the metal during the etch process. The ICP-RIEetching is performed using the following procedure. GaN etch isaccomplished using ICP etching with a Cl₂/N₂/Ar (25:5:2) gas mixtureunder a pressure of 5 mTorr with varying ICP etching power and radiofrequency (RF) power. For nitrides, Chlorine-based etches are usedbecause it has been shown to produce vertical sidewalls due to the ionassisted etching mechanism with smooth profiles. Temperature of the etchis a parameter that provides control of the sidewall angle. Withlow-temperature etch, the sub-surface damage may also be controlled.

Each sample is treated with a standard RCA clean before the activationannealing, the etching, and the measurements. Etching profile andsurface morphology may be investigated by SEM. The surface chemicalproperties of semiconductor after the etch is characterized using an XPSsystem and PL measurements performed at room temperature. The electricalproperties of etched semiconductor backbone are characterizedphotocurrent measurements. Photocurrent intensity is a direct measure ofthe surface recombination, i.e., higher photocurrent intensity willindicate less surface defect non-radiative recombination, hence lesssub-surface damage. For GaN, Ti/Al/Ti/Au (70 nm/70 nm/50 nm/50 nm) ohmicelectrodes are formed at both ends of the backbone nanostructures andthen annealed at temperatures from 500 C to 800 C for ˜1 min. Thenanodevices are then functionalized with different metal and metal-oxidenanoclusters using reactive sputtering.

A schematic representation of an exemplary fabrication flow forsemiconductor-nanocluster based gas sensors according to the presentinvention is shown in FIG. 37 . As shown, the fabrication flow providesfor parallel architecture, with multiple parallel sections. Themulti-analyte arrays can be created on one single chip (10 mm×10 mm) bydepositing clusters of different components on different micro-scaledevices. This is possible due to low-temperature sputtering process usedfor the cluster deposition. An array of multiple sensors (e.g. fordetecting NO_(x), SO_(x), CO_(x), NH₃, and H₂O) may be fabricated all onone single chip. FIG. 38 shows exemplary inter-digitated GaN devices onSi and sapphire substrates formed using top-down processes (e.g., suchas shown in FIG. 37 ).

Example 5

Protection against explosive-based terrorism may be achieved bylarge-scale production of nano-sensor arrays that are inexpensive,highly sensitive and selective with low response and recovery times. Inthis study, the selective response of GaN nanowire/TiO₂ nanoclusterhybrids to nitroaromatic explosives, including trinitrotoluene (TNT),dinitrotoluene (DNT), nitrotoluene (NT), dinitrobenzene (DNB) andnitrobenzene (NB) at room temperature is demonstrated. The sensorsdetected between 0.5 ppb and 8 ppm TNT with good selectivity againstinterfering compounds such as toluene. The sensitivity of 1 ppm of TNTis ≅10% with response and recovery times of ≅30 seconds.

N-type (Si doped) GaN nanowires functionalized with TiO₂ nanoclusterswere utilized for selectively sensing nitro-aromatic explosivecompounds. GaN is a wide-bandgap semiconductor (3.4 eV) with uniqueproperties. Its chemical inertness and capability of operating inextreme environments (high-temperatures, presence of radiation, extremepH levels) is highly desirable for sensor design. TiO₂ is aphotocatalytic semiconductor with bandgap energy of 3.2 eV (anatasephase). The TiO₂ nanoclusters were selected to act as nanocatalysts toincrease the sensitivity, lower the detection time, and enable theselectivity of the structures to be tailored to a target analyte (e.g.,the most common explosives, trinitrotoluene (TNT) and othernitro-aromatics).

Materials and Methods

GaN nanowires were grown by Molecular Beam Epitaxy method as describedin Bertness K A et al. (2008), supra, J. Crystal Growth310(13):3154-3158. The nanowires are aligned on a pre-patternedsubstrate using dielectrophoresis. Details of the device fabrication arereported in Aluri G S et al. (2011) “Highly selective GaN-nanowire/TiO₂-nanocluster hybrid sensors for detection of benzene and relatedenvironment pollutants,” Nanotechnology 22(29):295503. After fabricationof two-terminal GaN NW devices, the TiO₂ NCs were deposited on the GaNNW surface using RF magnetron sputtering. The deposition was done at325° C. with 50 standard cubic centimeters per minute (sccm) of Ar flow,and 300 W RF power. The nominal deposition rate was about 0.24 Å/s.Thermal annealing of the complete sensor devices (GaN NW with TiO₂nanoclusters) was done at 700° C. for 30 seconds in a rapid thermalprocessing system. The device substrates, i.e., the sensor chips, werewire-bonded on a 24 pin ceramic package for the gas sensingmeasurements.

The microstructure and morphology of the sputtered TiO₂ films used forthe fabrication of the sensors were characterized by high-resolutiontransmission and scanning transmission electron microscopy (HRTEM/STEM),selected-area electron diffraction (SAED), and field-emission scanningelectron microscopy (FESEM). For the TEM characterization, the GaN NWswere dispersed on 10 nm thick carbon films supported by Mo-mesh grids,followed by the deposition of TiO₂ NCs and annealing and subsequent Ptdeposition. The samples were analyzed in a FEI Titan 80-300 TEM/STEMmicroscope operating at 300 kV accelerating voltage and equipped withS-TWIN objective lenses, which provided 0.13 nm (STEM) and 0.19 nm (TEM)resolution by points. The instrument also had a Gatan CCD imageacquisition camera, bright-field (BF), ADF and high-angle annulardark-field (HAADF) STEM detectors to perform spot, line profile, andareal compositional analyses using an EDAX 300 kV high-performance Si/LiX-ray energy dispersive spectrometer (XEDS).

The as-fabricated sensors were placed in a custom designed gas chamberfor gas exposure measurements. Detailed description of the experimentalsetup and experimental conditions is provided in Aluri G S et al.(2011), supra, Nanotechnology 22(29):295503. The device characterizationand the time dependent sensing measurements were done using an AgilentB1500A semiconductor parameter analyzer. The gas sensing experimentswere performed by measuring the electrical conductance of the devicesupon exposure to controlled flow of air/chemical mixture in presence ofUV excitation (25 W deuterium bulb operating in the 215 nm to 400 nmrange). For all the sensing experiments with chemicals, breathing air(<9 μmol/mol of water) was used as the carrier gas. After the sensordevices were exposed to the aromatic compounds, they were allowed toregain their baseline current with the air-chemical mixture turned-off,without purging or evacuating the test-chamber.

Results

Morphological and Structural Characterization of NWNC Hybrids

TEM imaging was conducted under minimal beam intensity conditions closeto the Scherzer defocus at highest available accelerating voltage of 300kV using both stationary beam (bright-field TEM/SAED, phase-contrasthigh-resolution TEM) and scanning beam (STEM/XEDS) modes. Areas foranalyses were selected near the wire's edges and on the amorphous carbonsupport film in the vicinity of the NWs. FIG. 39 shows HRTEM micrographsof a GaN NW on a thin amorphous carbon support films with TiO₂ coating.The deposited TiO₂ layer formed an island-like film, where 10 nm to 50nm partially aggregated particles (circled areas in FIG. 39 ) were ofteninterconnected into extended two-dimensional networks. This wasconsistent with SAED and compositional analyses of deposited TiO₂ filmsindicating a mixture of polycrystalline anatase and rutile phases.Despite the limited contrast difference between TiO₂ and GaN, detailedHRTEM and HR-STEM observations revealed 0.35 nm (101) hcp latticefringes belonging to anatase.

Current-Voltage (I-V) Characteristics of NWNC Hybrids

Referring to FIG. 40 , I-V characteristics of a GaN NW two-terminaldevice at different stages of processing are shown. The I-V curves ofthe as-deposited devices were non-linear and asymmetric (with a lowcurrent of 35 nA). However, the current increased (to a 100 nA) with thedeposition of TiO₂ nanoclusters. This may be attributed to decreasedsurface depletion of the GaN NW due to passivation of surface states,and/or the high temperature deposition (325° C.) of the nanoclustersinitiating ohmic contact formation. The devices annealed at 700° C. for30 seconds showed significant changes in their I-V characteristics witha majority of the devices exhibiting linear I-V curves. This isconsistent given low resistance ohmic contacts to the nitrides requireannealing at 700° C.-800° C.

Sensing Behavior of GaN/TiO₂ NWNC Hybrid Sensors

The photocurrent through the bare GaN NW devices did not change whenexposed to different chemicals mixed in air, even for concentrations ashigh as 3%. In contrast, the TiO₂-coated hybrid devices responded evento the pulses of 20 sccm airflow in the presence of UV excitation. Theresponse of the TiO₂ NC-coated GaN nanowire hybrid sensors to differentconcentrations of benzene, toluene, ethylbenzene, chlorobenzene, andxylene in air is discussed above. The GaN/TiO₂ hybrids showed noresponse when exposed to other chemicals such as alcohols, ketones,amides, alkanes, nitro/halo-alkanes, and esters.

The response of the TiO₂ coated hybrid devices when exposed to aconcentration of 100 ppb of the aromatics and nitro-aromatics in air canis shown in FIG. 41 , plate (a). The photocurrent for these sensorsincreased with respect to air when exposed to toluene vapors, whereasfor every other aromatic compound the photocurrent decreased relative toair. The response is observed to increase with the increase in thenumber of nitro groups attached to the aromatic compound. The responseof the hybrid device to different concentrations of TNT in air from 8ppm down to as low as 500 ppt is shown in FIG. 41 , plate (b). Theresponse time is defined as the time taken by the sensor current toreach 90% of the response (I_(f)−I₀) when exposed to the analyte. TheI_(f) is the steady sensor current level in the presence of the analyte,and I₀ is the current level without the analyte, which in this case isin the presence of air. The recovery time is the time required for thesensor current to recover to 30% of the response (I_(f)−I₀) after thegas flow is turned off. The response and recovery times of thenano-devices to different concentrations of TNT are ≅30 seconds. Theresponse and recovery times of the rest of the compounds varied from ≅60seconds to ≅75 seconds.

The sensitivity is defined as (R_(gas)−R_(air))/R_(air), where R_(gas)and R_(air) are the resistances of the sensor in the presence of thechemical-air mixture and in presence of air, respectively. Thesensitivity plot of a hybrid device for the different aromatics andnitro-aromatics tested is shown in FIG. 42 . The sensitivity((R_(gas)−R_(air))/R_(air)) for 1 ppm of TNT is ≅10%. The devicesexhibit a very highly sensitive and selective response to TNT whencompared to interfering compounds like toluene. Toluene shows anincrease in response with respect to air, whereas TNT shows a decreasewhen compared to air. The plot identifies the sensor's ability to sensewide concentration ranges of the indicated chemicals. The sensitivity oftwo different devices (device 1—D1; device 2—D2) to the differentaromatic compounds can be seen in FIG. 43 .

As discussed above, oxygen vacancy defects (Ti³⁺ sites) on the surfaceof TiO₂ are the “active sites” for the adsorption of species likeoxygen, water, and organic molecules. In the presence of UV excitationwith an energy above the bandgap energy of anatase TiO₂ (3.2 eV) and GaN(3.4 eV), electron-hole pairs are generated both in the GaN NW and inthe TiO₂ cluster. Photogenerated holes in the nanowire tend to diffusetowards the surface due to surface band bending. This effect ofseparation of photogenerated charge carriers results in a longerlifetime of photogenerated electrons, which in turn enhances thephotoresponse of the nanowire devices in general. Since thenitro-aromatic compounds are highly electronegative, they tend toattract electrons from other molecules through charge transfer. Thischarge transfer between the adsorbed species on the TiO₂ nanocluster,and the nitro groups in the nitro-aromatic compounds increases the widthof the depletion region in the nanowire device, reducing the current.

The potential of the disclosed nanostructure-nanocluster hybrids fornext-generation nano-sensors having the capability to detect explosivecompounds quickly and reliably is clearly demonstrated. The GaN/TiO₂nanowire nanocluster hybrid devices tested detected trace amounts ofaromatic and nitro-aromatic compounds in air at room temperature withvery low response and recovery times (≅30 seconds). The nitro-aromaticexplosives like TNT are selectively detectable even for concentrationsas low as 500 ppt.

Example 6

Nitrogen dioxide (NO₂) sensors using gallium nitride (GaN) nanowires(NWs) functionalized with titanium dioxide (TiO₂) nanoclusters aredemonstrated. Exemplary sensor fabrication methodologies are describedabove (e.g., see Example 1 & FIG. 19 ).

FIG. 44 , plate (a) illustrates the dynamic responses of the TiO₂ basedsensor exposed to 250 ppm NO₂ mixed with breathing air under UVillumination and dark, and at room temperature. For each cycle, the gasexposure time was 300 s. FIG. 44 , plate (b) illustrates change inresistance under UV at mixtures of 100 ppm, 250 ppm, and 500 ppm withbreathing air, with the inset showing the measured responses under UV asa function of NO₂ concentrations with uncertainty. Sensitivity S ispresented by (I_(g)−I_(α))×100/I_(α), wherein I_(g) is the devicecurrent in the presence of an analyte in breathing air and Iα is thecurrent in pure breathing air, both measured 300 s after the flow isturned on. FIG. 45 illustrates schematically an NO₂ gas sensingmechanism of the TiO₂ sensor under UV illumination and at roomtemperature. FIG. 45 , plate (a) shows the mechanism in a darkenvironment with breathing air in. FIG. 45 , plate (b) shows themechanism under UV illumination in breathing air. FIG. 45 , plate (c)shows the mechanism under UV illumination with a mixture of NO₂ andbreathing air.

The response of the TiO₂ based sensor exposed to 500 ppm NO₂ under UVillumination and under dark at room temperature is shown in FIG. 46 . Asdescribed, the UV illumination allows for efficient photodesorption ofadsorbed oxygen and hydroxyl species, thus introducing additional oxidesurface sites or receptors for adsorption of target molecules.Photocatalytic reactions then occur between the adsorbed targetmolecules and the photo carriers in the oxide sites, leading to amodification of the surface potential and semiconductor backbone currentchange (transduction). The photodesorption of the adsorbed targetmolecules and reaction species leads to a reversal of photocurrent tobaseline (recovery).

A GIXRD scan of thermally processed ultrathin TiO₂ film is shown in FIG.47 , plate (a). Optical properties (bandgap) are illustrated in FIG. 47, plate (b).

Example 7

Carbon dioxide (CO₂) sensors using gallium nitride (GaN) nanowires (NWs)functionalized with tin oxide and copper oxide (SnO₂—CuO) nanoclustersare demonstrated. Exemplary sensor fabrication methodologies aredescribed above.

FIG. 48 , plate (a) illustrates schematically a SnO₂—Cu nanocluster CO₂sensor, including an electrode disposed on a sapphire substrate, and GaNNWs functionalized with SnO₂ nanoclusters and SnO₂—CuO nanoclusters. AFMimages of the SnO₂—Cu nanocluster CO₂ sensor are shown in FIG. 48 ,plates (b) and (c). FIG. 49 illustrates the dynamic response of theSnO₂—Cu based sensor exposed to CO₂ at room temperature for variousconcentrations. FIG. 50 illustrates graphically the response of the SnO₂based sensor at different relative humidity (RH) concentrations at roomtemperature.

Single Package Optically-Activated Gas Sensors

As described in the Background section, it would be desirable to providesystems, methods and devices which address the deficiencies of, e.g.,currently manufactured and packaged optically-activated gas sensors.Prior to describing embodiments associated with single package gassensors, examples of chips, sensing elements and sensor structures whichcan be used inside of the single package gas sensors will first bedescribed.

According to an embodiment, chip-scale gas sensors can be two-terminalphotoconductors, surface functionalized with nano-scale photocatalyticmaterial. An example of a sensor 6300 used for gas sensing is shown inFIG. 63 . This sensor 6300 includes eight individually addressablemicrosensors 6302 as well as various calibration and compensationelements 6304. The sensor 6300 can have, for example, a trapezoidalcross-section which can have a top-width of ˜250 nm and is formed bytop-down etching. Various combinations and quantities of theindividually addressable microsensors 6302 can be used as desireddepending upon the application the sensor 6300 is to be used in. Formore information regarding the chips (or dies) including materials andthe manufacturing of the chips, the interested reader is directed to thevarious documents incorporated herein by reference.

According to another embodiment, a multi-gas sensor for detecting thepresence of multiple gases or compounds can also be used in asingle-package sensor. An example of a multi-gas sensor which can beused in a single-package sensor is shown in FIG. 64 . FIG. 64 shows adie 6400 which has a plurality of sensors for detecting the presence ofdifferent compounds, more specifically there is a NH₃ sensor 6402, a COsensor 6404 and a H₂S sensor 6406. Additionally, there can be more thanone sensor of each type as represented by the redundant sensors 6408.

As described above, there are many different options for what type ofmaterials to create the gas sensors from depending upon the desired enduse of the gas sensors. An overview of an example of a process resultingin a single package gas sensor is now described with respect to the flowdiagram 6500 shown in FIG. 65 . Initially, in step 6502, a sensor designis selected based upon the desired end use. Using, for example, theprocesses described in the '862 patent or the other related, referencedpending patent applications, a wafer is processed to include etching andfunctional material deposition to create a gas sensor die in step 6504.In step 6506, the gas sensor die and an LED source are inserted into asingle package.

Of particular interest in this application, are embodiments associatedwith various facets of the single package to include, but not be limitedto, integration, disposition of components, filtering, assembly,materials and packaging. Embodiments addressing these areas aredescribed below in more detail.

According to an embodiment, an example of a single gas sensor packageprior to assembly completion is illustrated in FIGS. 66(a) and 66(b).FIG. 66(a) shows a lid 6600 with an optional filter 6602. FIG. 66(b)shows an exterior housing 6604 in which there is a sensor die 6606 whichacts as a base upon which other components are located. Disposed on thesensor die 6606 there is a gas sensor die 6608, and an LED die 6610.which can be a UV light source of varying wavelength. Additionally, invarious places on the various dies, there are a plurality of electricalconnections 6612 for connecting the components which need power to aplurality of electrical pads 6614.

When the LED die 6610 is a UV LED die, various options exist withrespect to power output, power usage, size and emitted wavelength foruse depending upon the application. For example, for one application theLED die emits light with a peak wavelength of 365 nm. However, for otherapplication, different wavelengths of emitted light can be preferred.Additionally, multiple LED dies which emit different wavelengths oflight can be disposed on a single sensor die 6606 in support of, forexample, activating gas sensor dies which are designed to detectdifferent types of gasses. This can provide data which can improve suchfeatures of the single package gas sensor as accuracy, detection limit,detection speed, etc. According to another embodiment, the intensity ofthe emitted light can be varied in a real time fashion based on, forexample, input from the user of the gas sensor.

According to an embodiment, regarding the wavelength of the emittedlight from the LED die 6610, it is too be understood that varyingwavelengths can be used depending upon the composition of the materialof the photoactivatable portion of the gas sensor die 6608, e.g., theemitted light can be in the range of ultraviolet light (100 nanometers(nm)) to roughly the end of the infrared spectrum (approximately 1millimeter (mm)). Additionally, according to an embodiment, a desireddiffusion of the emitted light from the LED die 6610 is where themaximum photon flux is reflected on the sensor die for the lower amountof optical power emission from the LED.

According to another embodiment, one can package more LED dies 6610 andalso more gas sensor dies 6608 as well as providing the LED dies 6610with different wavelength emission capabilities on the same packagegiving the ability to package different types of gas sensors on singlepackage. Alternatively, a single package can include LED dies 6610 ofdifferent sizes and power requirements. The LED die 6610 can include abuilt-in Zener diode for both electrostatic discharge protection andreverse bias protection.

According to an embodiment, FIG. 67 shows an image of a single packagegas sensor 6700, which includes the gas sensor die and the UV LED die(not shown in this image as they are located within the housing). Thesingle package gas sensor 6700 as shown has a lid 6702, a housing 6704and at least one gas ingress location 6706 on the lid 6702. According toan embodiment, the gas ingress location 6706 can be fitted with a filterto reduce dust, for use in humid conditions, condensing humidity as wellas providing general protection for the internal dies. According to anembodiment, the filter can also be an active filter in that the filterprovides filtering of different molecules to provide more selectivity ofthe sensing device. For example, the gas sensor is designed to detectCO₂ but the gas sensor has some cross sensitivity to CO, the filter canbe designed to filter CO while not impeding or perhaps minimallyimpeding the CO₂. Additionally, according to an embodiment, the lid 6702is made of a material which can reflect, at least internally to the gassensor, light emitted from LED die 6610. The lid 6702 can be flat orhave a dome shape. Examples of such a material for the lid 6702 includealuminum, steel and tin. Examples of materials from which the housing6704 can be made include plastic, Teflon, ceramics, various metals andcombinations of these items, e.g., a Teflon coating on a metal.

FIGS. 68-70 show additional sketches of examples of portions of thesingle package gas sensor. More specifically, FIG. 68 shows the housing6802, the sensor die 6804, the gas sensor die 6806 and the UV LED die6808. In this example, the location of the UV LED die 6808 relative tothe gas sensor die 6806 is selected to represent the location of the UVLED die 6808 such that when powered to emit UV light, the UV light isreflected off of the lid such that the gas sensor die 6802, and theirassociated individually addressable microsensors 6302 (not shown in FIG.68 ), are activated by the emitted, reflected UV light.

According to an embodiment, gas sensor die 6806 (or in some casesmultiple gas sensor dies) are present and activated by receiving UVlight. This light is typically reflected off of the lid prior to thelight being received by the gas sensor die 6806. Activating, in thiscontext, describes the active sites of the gas sensor die 6806 areenabled to detect a target analyte only when receiving the UV light.Regarding a placement relationship of the LED die 6808 and the gassensor die 6806, the LED die 6808 can be placed next to the gas sensordie 6806, at a corner of the gas sensor die 6806, mounted directly ontop of the gas sensor die 6806 or at another location which provides forthe desired emission, reflection and diffusion of the light foractivation of the gas sensor die 6806.

The LED die 6808 can be further mounted using “flip-chip” bonding, wherein the LED die is flipped on its bottom surface and mounted directly onthe sensor die, allowing to LED to excite the catalytic surfacedirectly, without the use of reflecting lid, further simplifying thepackaging, making the package smaller and lower cost as the entirepackage can be made from plastic. Further, the relative position of theplace of the LED die 6808 to the gas sensor die(s) 6806 can depend onmany factors, such as, shape and size of the package, lid material, gassensor die 6806 size and shape thickness, LED power and LED die 6808size, and thickness, and can be tailored depending on the applicationalso.

Additionally, while not shown in FIG. 68 there can be a plurality of gassensor dies 6806 all of which would be activated by the reflected UVlight. FIG. 69 shows a lid 6900 with a gas ingress point 6902 and thehousing 6802 prior to the lid 6900 and the housing 6802 being assembledtogether. FIG. 70 shows the single gas sensor package 7000 and a view ofthe single gas sensor package's cross section 7002. The lid 6900 and thehousing 6802 can be attached, for example, through the use of anadhesive. However, other attachment methods can also be used as desired.

According to an embodiment, the relative position of the LED die(s) 6808and the gas sensor die(s) 6806 are selected to allow for a desireddiffusion of the reflected light. This occurs when the desired diffusionoccurs where a maximum, or near maximum, photon flux is reflected ontothe gas sensor die 6806 using a lowest, or relatively low, effectiveamount of optical power emission from the at least one UV LED.Physically this can be implemented as shown in FIGS. 71 and 72 . FIGS.71 and 72 show placement the LED die 6808 and the gas sensor die 6806 onthe sensor die 6804 as well as a representation of the gas ingresslocation 6706. In FIGS. 71 and 72 it is noted that when looking down onthe sensor die 6804, for increasing or maximizing photon flux reflectedonto the gas sensor die 6806, the placement of the gas ingress location6706 is in such a place as to not be overtop of either the gas sensordie 6806 or the LED die 6808 as this can reduce reflectivity. FIGS. 73and 74 show undesirable setups as the gas ingress location 6706placement overlaps either the gas sensor die 6806 or the LED die 6808when looking down upon the gas sensor 6804. Further, according to anembodiment, these concepts can also be implemented, or implemented asbest able, when a plurality of one or more of the gas sensor dies 6806,the LED dies 6808 and the gas ingress locations are present.

As described above, embodiments describe a gas sensor die and a UV LEDboth disposed on a same, single chip. The gas sensor die used can be asingle gas sensor or a multiple gas sensor on a single chip. Variousdifferent gas sensors can be used within the described packaging. Forexample, one product can be a NO₂ and CO₂ sensor on one chip as anindoor air quality monitoring device. For another example, anotherproduct could be a Cl₂ and HCl sensor for toxic industrial chemicaldetection. Other sensor examples include, but are not limited to,H2/Methane for explosive gas sensing, H2S/CO for wastewater treatment,NH3 for cold storage monitoring and H2 for fuel cell based applications.Additionally, other sensing options, as described above and in theincorporated by reference documents, can also be used in the singlesensor package. Advantages include, but are not limited to, providinggas sensing on a chip-scale package, reduced costs compared totraditional sensors, sensors which can selectively detect compoundswithout being effected by the presence of other interfering gases,sensors which are robust with respect to temperature and humiditychanges, sensors which have reduced degradation when exposed tocorrosive gases and the ability to provide multi-gas detection on asingle chip device.

According to an embodiment, while this description of the packaging hasfocused on single package gas sensors, it is too be understood that thesingle packaging concept can be implemented with other types of sensor,particularly where a sensor and a light (or other type of emitter)source are to be in a single package. For example, some alternativeapplications can include UV/photodetection, radiation detection,temperature sensing, humidity sensing. Similar architecture andmodifications can also be used for detection of specific molecules in aliquid medium, such as, mercury, lead, other heavy metals in water orother liquid mediums. According to another embodiment, a modifiedpackaging architecture with sensor(s) can also be used for detection ofbiomarkers in blood, detection of biological agents, e.g., viruses andbacteria, in a liquid medium as well as detection of chemical species ina liquid medium such as water, blood and other biological medium.

According to an embodiment, a control logic can be implemented by aprocessor, microprocessor and the like, for managing the powerconsumption of the LED die 6806, as well as the entire single packagegas sensor 7000, as well as response time and recovery times. LEDintensity for some detectable gasses is related to response and recoverytimes. This means that higher intensity LED light can make the sensorsense faster and to also refresh faster which, in some scenarios, couldlead to being able to sense smaller amounts of the desired chemical inatmosphere that is lower than its traditional detection limit. However,as increasing the LED intensity increases total power consumption, soembodiments also allow for real time LED intensity variation by anoperator to optimize the single package gas sensor 7000 to the user'sneeds.

According to an embodiment, there is a method for assembling a singlepackage gas sensor as shown in the flowchart 7100 of FIG. 75 . In step7102, disposing a substrate in a housing; in step 7104, disposing atleast one gas sensor die on the substrate; in step 7106, disposing atleast one ultraviolet (UV) light emitting diode (LED) on the substrate;and in step 7108, attaching a lid to the housing, wherein the lidincludes at least one gas ingress point.

Application Specific Integrated Circuit (ASIC) Controllers for GasSensors

Embodiments describe using an ASIC as a controller for controllingoperation of the afore-described sensors (or other sensors), optionallyin a single package (as described above). One of the reasons that acontroller is needed for gas sensors such as those described above isthat the fabricated bridge sensors will have an unknown baselineresistance level which can vary by thousands of kΩ. This causes aninitial offset in the differential bridge voltage which is thenamplified by the analog front end of the chip. A change in sensor outputwill thus be limited if the offset is large and causes the signal tosaturate, providing less accurate gas concentration readings. Thusaccording to these embodiments, the ASIC is designed to automaticallycalibrate out any offset such that the sensor output starts at mid-railor zero.

A block diagram of an ASIC controller according to an embodiment isshown in FIG. 76 . Therein, the ASIC 7600 is electrically connected to aUV LED 7602 and gas sensing elements 7604, e.g., wherein the UV LED 7602provides UV light onto the gas sensing elements 7604 to actuate the gassensing elements 7604 as described above. Internal to the chip 7600,control of the components is handled by an internal controller 7605which is disposed on the chip 7600. Optionally, as described in moredetail below, the ASIC 7600 may also be connected to an externalmicrocontroller 7606 to control on-chip operation. According to thisembodiment, an ASIC 7600 includes a number of components, for example,an analog front-end (AFE) which includes offset cancellation elementsand amplification of bridge sensor outputs, a successive approximationanalog-to-digital converter (ADC) 7610, a clock generator 7612, an LEDdriver 7614, and sensor supply voltage regulator 7616. The functionalityof these components will be described in more detail below. As will beappreciated by those skilled in the art, ASIC 7600 will also includeother elements which are not shown in FIG. 76 , e.g., othermiscellaneous circuits including on-chip bias generators and buffers,and digital logic.

In operation, voltage regulator 7616 sources power to the gas sensingelement 7604 (also sometimes referred to herein as “chemiresistors”.Periodically, the LED driver 7614 drives the UV LED 7602 to stimulatethe chemiresistors 7604, under control of the PWM control signalgenerated by internal controller 7605 to control the duty cycle of theLED driver signal, and then the resistance of those sensing elements7604 upon exposure to the UV light is measured. To this end, the LEDdriver 7614 can include a constant-current driver (not shown) which isdesigned to sink 10 mA of current and an amplifier (not shown) which isused to modulate the gate voltage of a PMOS such that its drain voltagematches a predefined reference level. The resulting gate voltage isbuffered to a large PMOS which acts as the LED current sink in serieswith a large NMOS transistor that switches on and off with the PWMsignal. A PWM driver is used to increase the transition-time of the PWMsignal to increase stability. In this embodiment, there are four bridgepairs 7607 of chemiresistors with plus and minus channel outputs fromeach bridge pair (i.e., IN1+/IN1−, IN2+/IN2−, IN3+/IN3− and IN4+/IN4− inFIG. 76 ) via which the resistance of each gas sensor in the set of fouris measured.

Under the control of the internal controller 7605 via the Pixel addr.signal line, one of the four channels is selected by multiplexer 7618for input to the amplifier 7608 in the analog front end of the chip7600. The gain level for the amplifier 7608 is set by the calibrationcontrol signal from internal controller 7605 to provide calibration ofthe gas sensors' outputs as will be described in more detail below. Amore detailed view of this portion of the ASIC 7600's circuitryaccording to an embodiment is provided in FIG. 77 . Therein, a 3 op ampinstrumentation amplifier topology was chosen for amplifier 7608 due toits small size, simplicity, and ability to achieve relatively low gains.As seen in dotted block 7608, it consists of three op amps 7700, 7702,7704, three pairs of resistors 7706, 7708, 7710, and a bank of fourresistors 7712 that can be programmed to set the overall amplifier gain.The four pairs of gas sensor inputs are multiplexed using an analog mux7618 and fed into the instrumentation amplifier 7608.

As mentioned previously, the amplifier 7608's output needs to becalibrated at the start of a measurement cycle. This is because thefabricated gas sensors 7604 will have varying initial resistances whichlead to an unknown amplifier output. So in order to calibrate eachsensor, the ASIC 7600 evaluates the output of the amplifier 7608 whenthe sensor is not exposed to any gas and marks that output as thezero-start level. Depending on the final use case, the ASIC 7600 canthen adjust the zero-start level to be near 0 V, or at the middle of thesupply rails (V_(dd)/2 V). Choosing this target value is done by settingV_(gnd) (at the positive input of the comparator 7710 seen in FIG. 77 ).The actual adjustment is made by switching on and off a bank of currentsources 7712 at the V_(ref) node of the instrumentation amplifier 7608.These switches are controlled using digital logic 7714 that uses aSAR-like binary search algorithm to iteratively switch on the currentsources 7712 from largest to smallest. The decision to keep or discardeach current source is set by the comparator's output. As such thecomparator 7710 is designed to discriminate small differences in itsinputs and able to operate near the supply rails.

The output of the analog front end 7608 is digitized, according to thisembodiment, using a 12 bit successive approximation analog-to-digitalconverter (ADC) 7610. As seen in FIG. 78 , which extends on elementsillustrated in FIG. 77 , the signal (stored in sample and hold 7800)being digitized is compared at comparator 7801 to the output of adigital to analog converter (DAC) 7802 whose value is controlled by theSAR logic 7714. The output of the DAC 7802 is changed in a binary searchfashion by logic 7714 until it converges to the value of the inputsignal. Multiplexer 7806 switches between the aforementioned evaluationof the current source decisions and the comparison between the ADC/DACsignals toward the logic block 7714.

Regarding the internal controller 7605, this controller can operate ineither an internal control mode or an external control mode (if theoptional external controller is used). When operating in internalcontrol mode, after calibration, the internal controller 7605 waits aspecified sampling interval, activates the PWM control signal toactivate the LED driver 7614 and UV LED 7602, waits for a ramp-upduration, then takes a measurement of each of the four inputs, flagsmicrocontroller to read, then goes to a wait mode until the nextsampling interval. When operating in external control mode, aftercalibration, the internal controller 7605 goes into a wait mode, anexternal trigger can be sent by the external microcontroller 7606 toinitiate a read of all four inputs and the measurements are saved intoregisters. The external controller 7604 can then read the registersthrough the I2C bus.

As can be seen in FIG. 76 , other than the UV LED 7602 which is used toactivate the sensing elements 7604, the sensing elements 7604 themselvesand the optional external microcontroller 7606, all of the componentsneeded to control the UV LED 7602 and to calibrate the sensing elements7604 are disposed on a single chip 7600. Regarding the optionalmicrocontroller 7606, if provided, this element enables the ASIC 7600 toreceive instructions externally for calibrating the amplifier 7608 andoperating the UV LED 7602. Providing an external microcontroller 7606enables, for example, the gas sensors to 7604 to be queried for gasrelated data at a time and manner which is controlled by an externalentity. The arrows shown in FIG. 76 between the external microcontroller7606 and the internal controller 7605 represents signals conveyedbetween the two controllers via, for example, an I2C bus. For example,the SCL and SDA signals represent the clock and data signals transmittedby the external microcontroller 7606 to the internal controller 7605 viathe I2C bus. The DRDY signal flags the internal controller 7605 to readthe data. The CLK_SEL signal allows the microcontroller 7606 to instructthe ASIC 7600 to either use its internally generated clock signal (viaclock generator 7612) or to use the clock signal sent by microcontroller7606 for clocking the various electrical components on the chip. Lastly,the RESET signal enables microcontroller 7606 to reset the values usedby chip 7600 to baseline values.

From the foregoing, it will be appreciated that some embodiments includethe following features: (a) each of the four sensor inputs hasindividual calibration settings since it is unlikely that multiplesensors will have identical initial characteristics and, therefore,during a readout, the calibration settings are loaded separately foreach sensor; (b) each of the four sensor inputs has individual gainsettings; and (c) each of the four sensor inputs has its own individualzero-level setting.

In addition to physical chips or ASICs, embodiments can also beexpressed as methods, an example of which is provided in the flow chartof FIG. 79 . Therein, a method 7900 for operating an ApplicationSpecific Integrated Circuit (ASIC) controlling one or more gas sensorsincludes the steps of powering on the ASIC 7902 which generates a pulsewidth modulated (PWM) signal for driving at least one ultraviolet (UV)light emitting diode (LED), wherein an output of the at least one UV LEDactivates the one or more gas sensors; upon generating the PWM signal,entering a steady state 7904 prior to calibrating an output of anamplifier, wherein the amplifier receives one or more inputs associatedwith the one or more gas sensors; and performing calibration 7906 of theamplifier to remove offsets associated with outputs associated with theone or more gas sensors.

All publications and patents mentioned in this specification are hereinincorporated by reference to the same extent as if each individualpublication or patent application was specifically and individuallyindicated to be incorporated by reference in its entirety. While theinvention has been described in connection with specific embodimentsthereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth.

Although the features and elements of the present embodiments aredescribed in the embodiments in particular combinations, each feature orelement can be used alone without the other features and elements of theembodiments or in various combinations with or without other featuresand elements disclosed herein. The methods or flow charts provided inthe present application may be implemented in a computer program,software, or firmware tangibly embodied in a computer-readable storagemedium for execution by a general-purpose computer or a processor.

This written description uses examples of the subject matter disclosedto enable any person skilled in the art to practice the same, includingmaking and using any devices or systems and performing any incorporatedmethods. The patentable scope of the subject matter is defined by theclaims, and may include other examples that occur to those skilled inthe art. Such other examples are intended to be within the scope of theclaims.

1. A method for operating an Application Specific Integrated Circuit(ASIC) controlling one or more gas sensors, the method comprising:powering on the ASIC which generates a pulse width modulated (PWM)signal for driving at least one ultraviolet (UV) light emitting diode(LED), wherein an output of the at least one UV LED activates the one ormore gas sensors; upon generating the PWM signal, entering a steadystate prior to calibrating an output of an amplifier, wherein theamplifier receives one or more inputs associated with the one or moregas sensors; and performing calibration of the amplifier to removeoffsets associated with outputs associated with the one or more gassensors.
 2. The method of claim 1, further comprising: determiningwhether the ASIC is operating in an internal control mode or an externalcontrol mode.
 3. The method of claim 2, wherein when the ASIC isoperating in an internal control mode, the ASIC includes amicrocontroller for controlling logic associated with performing thecalibration of the amplifier and operating other functions of the ASIC.4. The method of claim 2, wherein when the ASIC is operating in anexternal control mode, the ASIC receives instructions from an externalsource for performing the calibration of the amplifier and operatingother functions of the ASIC.
 5. The method of claim 1, wherein the stepof performing calibration of the amplifier to remove offsets associatedwith outputs associated with the one or more gas sensors furthercomprises: adjusting a start level voltage output of the amplifier foreach of the one or more gas sensors such that the start level voltageoutput is either substantially zero volts or substantially a middle of asupply rail voltage based on a use case for each of the one or more gassensors.
 6. The method of claim 5, wherein the use case for each of theone or more gas sensors is associated with a specific gas.
 7. The methodof claim 3, wherein when operating in the internal control mode, themethod further comprises: after calibration, waiting a specifiedsampling interval; activating the PWM signal to activate the LED driver;waiting for a ramp-up duration; taking a resistance measurement of eachof the gas sensors; flagging reading of the resistance measurements andentering a wait mode until a next sampling interval.
 8. The method ofclaim 4, wherein when operating in the external control mode, the methodfurther comprises: after calibration, going into a wait mode; receivingan external trigger to initiate a read of all inputs; and savingmeasurements from the inputs into registers.
 9. The method of claim 1,wherein each of the one or more gas sensors has four inputs, whereineach of the four inputs has its own calibration settings, furtherwherein each of the four inputs has individual gain settings.
 10. Themethod of claim 1, wherein the ASIC is a part of a single package gassensor.
 11. An Application Specific Integrated Circuit (ASIC) configuredto control one or more gas sensors, the ASIC comprising: a lightemitting diode (LED) driver which receives a pulse width modulator (PWM)signal for driving at least one ultraviolet (UV) LED, wherein an outputof the at least one UV LED activates the one or more gas sensors; and ananalog front end and an analog to digital converter configured tocalibrate an output of an amplifier to remove offsets associated withoutputs associated with the one or more gas sensors, wherein calibrationof the amplifier occurs after both generation of the PWM signal andentering a steady state by the one or more gas sensors, further whereinthe amplifier receives one or more inputs associated with the one ormore gas sensors.
 12. The ASIC of claim 11, wherein it is determined ifthe ASIC is operating in an internal control mode or an external controlmode.
 13. The ASIC of claim 11, wherein when the ASIC is operating in aninternal control mode, the ASIC further comprises: a microcontroller forcontrolling logic associated with performing the calibration of theamplifier and operating other functions of the ASIC.
 14. The ASIC ofclaim 12, wherein when the ASIC is operating in an external controlmode, the ASIC receives instructions from an external source forperforming the calibration of the amplifier and operating otherfunctions of the ASIC.
 15. The ASIC of claim 11, wherein the amplifierfront end and the ADC are further configured to adjust a start levelvoltage output of the amplifier for each of the one or more gas sensorssuch that the start level voltage output is either substantially zerovolts or substantially a middle of a supply rail voltage based on a usecase for each of the one or more gas sensors.
 16. The ASIC of claim 15,wherein the use case for each of the one or more gas sensors isassociated with a specific gas.
 17. The ASIC of claim 13, wherein whenthe ASIC is operating in the internal control mode, the microcontrolleris further configured to: after calibration, waiting a specifiedsampling interval; activating the PWM signal to activate the LED driver;waiting for a ramp-up duration; taking a resistance measurement of eachof the one or more gas sensors; flagging reading of the resistancemeasurements and entering a wait mode until a next sampling interval.18. The ASIC of claim 14, wherein when operating in the external controlmode, the ASIC is further configured to: after calibration, going into await mode; receiving an external trigger to initiate a read of allinputs; and saving measurements from the inputs into registers.
 19. TheASIC of claim 11, wherein each of the one or more gas sensors has fourinputs, wherein each of the four inputs has its own calibrationsettings, further wherein each of the four inputs has individual gainsettings.
 20. The ASIC of claim 11, wherein the ASIC is a part of asingle package gas sensor.